.. . ....r.: 2 7. . C11,. . .: mmgtfl MICHIG STATE U Ill/I/////I///ll/lljl///I///fl7/7/7llli//lWill/WI! 3 129 00548 0326 LIBRARY Michigan State University This is to certify that the dissertation entitled Responses of Pickling Cucumber Plant: to Drought Stress During the Reproductive Growth Stage presented by Abdul Kader Janoudi has been accepted towards fulfillment of the requirements for Ph - D;_degree in lintticultmze . p r‘\\ x) v o :, «. p K . ’B/aCLTV‘t/‘KJ g. - CL) cox“- L 'L/ Dr. Irvin Widders Major professor Date_Eab4 6 , 1989 MS U is an A ffirmati ve Action/Equal Opportunity Institution 0-12771 _ _ .__._.__— —___~_____..__.. _—.»—. k7— RETURNING MATERIALS: Place in book drop to remove this checkout from your record. FINES will be charged if book is returned after the date stamped below. RESPONSES OF PICKLING CUCUMBER PLANTS TO DROUGHT STRESS DURING THE REPRODUCTIVE GROWTH STAGE BY Abdul Kader Janoudi A DISSERTATION Submitted to Michigan State University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Department of Horticulture 1989 l AW H‘ , ABSTRACT RESPONSES OF PICKLING CUCUMBER PLANTS TO DROUGHT STRESS DURING THE REPRODUCTIVE GROWTH STAGE BY Abdul Kader Janoudi Cucumber (Cucumis sativus L.) jplants have high water requirements for’ growth. and. development. Water' deficits during the fruiting growth stage reduce fruit yield and quality. This study was conducted to evaluate the tolerance and physiological responses of selected commercial pickling cucumber parental lines and Fl-hybrids to drought stress. Cucumber plants were grown in containers in the greenhouse and subjected to drought stress during the fruiting growth stage. Plants were rewatered, and another water deficit exposure initiated , when plant water potentials had reached -O.5 to -0.8 Mpa. Leaf sap osmolality was measured using a vapor pressure osmometer. Leaf gas exchange parameters were measured using an open gas exchange system with an infrared COZ-analyzer. In each experiment, individual fruit dimensions and fresh and dry weights were recorded. At the end of certain experiments, leaf area and dry weight and stem and root dry weights were measured. Carbon—dioxide assimilation rates (A) of drought stressed plants averaged 6.9 umol.m'zs'l as compared to 19.0 for well-watered plants. However, the adverse effects of water deficits on A were reversible. Within 12 hours of Abdul kader Janoudi being rewatered, stressed plants attained photosynthetic rates similar to those of well-watered plants. Only 36.5% of the decrease in photosynthetic rate in drought-stressed plants could be attributed to the decrease in Ci associated with stomatal closure. Under water-limiting conditions, fruiting plants maintained higher photosynthetic rates than non-fruiting plants. Cucumber plants allocated photoassimilates to developing fruits at the expense of vegetative plant parts. The 'magnitude of {osmotic adjustment in cucumber leaf tissue of stressed cucumbers ranged between 0.06 and 0.1 Mpa. Increases in K+ concentration in leaf lamina tissue could account for most of the observed decrease in osmotic potential under'idrought stress conditions” In ‘water stressed plants, leaf osmotic potential increased following rewatering. Drought stress reduced cucumber ‘vegetative growth and fruit set by 20.8% to 38.8% and 25.5% to 46.4%, respectively. Water deficits reduced fruit growth rates but did not alter the fruit bearing pattern of stressed plants. It was concluded that, under the experimental conditions of this study, the genotypes tested have a low drought tolerance. To My Family iv ACKNOWLEDGEMENTS I would like to thank a number of people and organizations for their assistance and support. I thank the members of my graduate committee -- Dr. Irvin Widders, advisor; James Flore; Dr. Kenneth Poff; Dr. Hugh Price and Dr. Maurice Vitosh -- for reviewing, editing and critiquing this study. I must express my sincere gratitude and appreciation for the advice, assistance and encouragement provided by my Major professor, Dr. Irvin Widders. His continuous support and concern as an advisor were major factors in helping me to complete my studies. Thanks also go to the Campbell Institute for Agriculture Research, Napoleon- Ohio, and to Vlasic Foods for their financial support and cooperation which made this study possible. The financial support provided by the Hariri Foundation during the first year of my studies at Michigan State University is also greatly appreciated. The efforts of Mr. Rafic Hariri and the Hariri Foundation in providing aid for Lebanese students can. not. be given enough appreciation. Finally, I would like to thank my wife for her continuous support and love, especially during the dark days of the war in Lebanon. TABLE OF CONTENTS List of Tables ............................ . ........ List of Figures..... ............................... List of abbreviations .............................. Introduction ......................... . ............. -Literature review ................ . ................. - Effects of water deficits on plant growth, development and yield ............. . .............. - effects on vegetative growth .................. - effects on fruit set and fruit quality ........ - Water requirements of cucumbers .................. - Strategies for dealing with water deficits ....... - Cultural practices.. ....... ....... ............. - soil management and irriagtion ................ - antitranspirants .............................. — early cultivars ............................... - Plant breeding ................................. - Plant adaptations to water deficits .............. - Stomatal adaptation ............................ - Root growth .................................... - Leaf rolling ................................... — Osmotic adjustment ............................. - Osmotic adjustment in plants.. ................... - Role in drought tolerance..... ................. — Solutes in osmotic adjustment.. ...... . ...... ... - Advantages and limitations...... ......... . ..... - Effects of water deficits on plant gas exchange characterisitcs and assimilate translocation ..... ~ Stomatal responses ...... . .......... . ....... .... - Photosynthesis ................................. - Water use efficiency ........................... - Translocation ............................ . ..... - Effects of fruiting on photosynthesis and assimilate allocation ............................ Chapter I. Effects of water deficits and fruiting on carbon assimilation and allocation in pickling cucumber plants. - Abstract.............. ......................... - Introduction................................... - Materials and Methods. ........ . ................ - Plant material. ................................ w KOO)GD\10\O\O\O\J>O)LJ 25 26 28 28 - Water regimen ................................... 29 - Deflowering ................................... 29 - Leaf gas exchange measurements ................ 29 - Results ............. . ............. . .............. 31 - Discussion ....................................... 45 - Literature cited ................................. 51 - Chapter II. Evidence for osmotic adjustment in leaf tissue of pickling cucumbers in response to drought stress. - Abstract..... ...... . ............................. 55 - Introduction .................. . .................. 56 - Materials and Methods ............................ 57 - Plant material .................................. S7 — Water deficit ................ ...... ............. 58 - Water potential determinations .................. 58 - Osmotic potential determinations ................ 58 - Heat girdling ........... . ........ . .............. 59 - Leaf sugar and potassium determinations ......... 59 - Results..... .............. ... ..... . ............... 60 - Discussion...... .......... . ....................... 69 - Literature cited................. ................. 73 - Chapter III. Water deficit effects on pickling cucumber plant growth and fruit productivity, growth and quality. -Abstract ......... . ............... . ................ 75 - Introduction ..................................... 76 — Materials and Methods ............... . ............. 77 - Plant material ..... . ............................ 77 - Water deficit ................... . ............... 77 - Genotype evaluation...... ..... . ......... . ....... 78 - Results................ ........................... 79 - Discussion. ...... . ........ . ........ . .............. 89 - Literature cited. ......... ....... ............. .... 94 - Summary and Conclusions .............................. 96 ~ Appendices - Appendix A. Diurnal changes in gas exchange para- meters of cucumber leaves.... ....................... 108 - Appendix B. Effect of leaf age on photosynthetic rate in cucumber leaves ............................. 110 - List of References ......... . ......................... 111 LIST OF TABLES Chapter I; Table 1. Effects of water regimen on gas exchange properties of greenhouse and field-grown cucumbers... Table 2. Water use efficiency and water status of greenhouse- grown cucumber plants following gas exchange measurements ................................ Table 3. Effects of water deficits and CO2 level on photosynthesis in cucumber leaves ................. Table 4. Effects of water deficit and fruiting on gas exchange parameters of cucumber leaves. .......... Table 5. Effects of water deficit and fruiting on dry matter production and partitioning in cucumber plants ............................................... Table 6. Soluble sugar levels in the 5th leaf from the shoot apex of drought stressed and non-stressed fruiting and defruited cucumber plants ............... Chapter _I Table 1. Leaf osmotic potentials of eight pickling cucumber genotypes exposed to drought stress or irrigated during fruit development....... ............ Table 2. Osmotic potentials of cucumber leaves during three exposures to drought stress under greenhouse conditions ..... O O O O O O O O O O O O . O OOOOOOOOOOOOOOOOOOOOOOOO Table 3. Leaf osmolality and concentration of selected solutes in cucumber leaf lamina tissue under drought stress and well watered conditions ........... Table 4. Changes in cucumber leaf osmotic potentials following relief from water stress ............. . ..... Table 5. Changes in solute concentrations in leaf lamina tissue of drought stressed cucumbers viii page 37 38 4o 41 43 44 61 62 65 66 ix following rewatering ................................. Table 6. Changes in solute relations in heat girdled leaves of drought stressed and well watered cucumber plants following rewatering .......................... Chapter III Table 1. Effects of drought stress on dry matter productivity in selected pickling cucumber genotypes (1986) ............................................... Table 2. Effects of genotype and water deficits on fruit productivity and quality of greenhouse-grown pickling cucumbers (1987) ............................ Table 3. Effects of water deficits on cucumber fruit dimensions ........................................... Table 4. Effects of water stress and fruiting sequence on cucumber fruit growth rates ....................... Appendix A Table 1. Net photosynthetic rates for leaves at different node positions on pickling cucumber plants. 67 68 81 82 87 88 110 LIST OF FIGURES Chapter I page Figure 1. Effects of increasing PPFD on photosynthetic rate of the 5th leaf from the shoot apex of cucumber plants. Ambient temperature and CO2 concentration were maintained at 25C and 350 ppm, respectively... 33 Figure 2. Correlation between stomatal conductance and leaf gas exchange parameters. Data points represent individual measurements on cucumber leaves at the 5th node from the shoot apex .............................. 34 Figure 3. Temperature response of CO assimilation rate of the 5th leaf on cucumber plants. Values are means of 4 measurements and vertical bars indicate standard error of the mean ............................................ 35 Figure 4. Influence of vapor pressure deficit on water use efficiency in cucumber leaves. Confidence limits at the 5% level are shown. Data points are from measurements on individual leaves in greenhouse and field plants.... 36 Chapter I I Figure 1. Effects of water regimen on fruit growth rate. Fruit volume was calculated from dailty measurements of fruit diameter and length and assuming a cylindrical fruit shape. Confidence limits at the 5% level are shown ...... . ............... . .......... ......... ........ Figure 2. Distribution of fruit harvest over a three-week harvest period. Fruits were harvested when 50 +/- 3.mm in diameter. G. and M. refer to gynoec1ous and monoec1ous flowering, respectively ................................ 85 Appendix 3 Figure 1. Diurnal changes in gas exchange parameters of cucumber leaves. Measurements were made on the 4th or 5th leaf from the shoot apex of greenhouse grown cucumber plants. A portable open gas exchange system was used. A HID sodium lamp was used as the lighg _1 108 s . source to provide light levels of >=1000 umol.m X xi Figure 2. Diurnal changes in gas exchange parameters of cucumber leaves. Measurements were made on the 4th or 5th leaf from the shoot apex of greenhouse grown cucumber plants. A portable open gas exchange system was used. A HID sodium lamp was used as the light source_ o_provide light levels of >= 1000 umol.m s ....................................... 109 LIST OF ABBREVIATIONS A . CO2 assimilation rate C : Degree celsius CA: Ambient carbon dioxide Ci : Intercellular C02 cc : Cubic centimeter cm : Centimeter C02 : Carbon dioxide ,dm : Decimeter g : Gram gS : Stomatal conductance HID : High Intensity Discharge hr : Hour H20 : Water IRGA : Infra-red gas analyzer K : Potassium K ’ Potassium ion kPa : Kilopascal l : Liter LD ratio: Length to diameter ratio L.S.D : Least significant difference M : Molar m : Meter mg : Milligram min : Minute ml : Milliliter mmolal : Millimolal MPa : Megapascal N : Nitrogen NaOH : Sodium Hydroxide P : Phosphorus PAR : Photosynthetically Active Radiation PPFD : Photosynthetic Photon Flux Density ppm : Parts per million s : Second VPD : Vapor pressure deficit wt : Weight WUE : Water Use Efficiency um : Micrometer umol : Micromole xii ,_._——z . Introduction Cucumbers are fleshy plants which have a high water requirement for growth and development. Machine harvested pickling cucumbers are mainly grown under rainfed conditions. In the mid-western United States, periods of drought, 7 to 10 days in duration, are common during the summer months, June through August, and lead to moderate to severe water deficits in rain-fed cucumber crops. Transient water deficits are also observed frequently in cucumber plants due to high transpirative water loss at mid day. Such water deficits result in temporary leaf wilting and stomatal closure. In many crops, stomatal closure has been found to result in reductions in photosynthetic rates. Decreases in cucumber fruit quality have been associated with the decrease in photoassimilate supply which can be expected under conditions of drought stress. The flowering and fruiting period has been identified as an important stress- sensitive growth stage in plant development as related to crop productivity. It was hypothesized that water deficits, during the reproductive growth stage, limit plant growth and decrease photosynthetic rates and that the combined effects of smaller leaf areas and lower C02 assimilation rates decrease fruit quality and productivity. 2 strategy of cucumber plants. Fruiting cucumber plants have higher photosynthetic rates than non-fruiting plants and allocate photoassimilates to fruits at the expense of vegetativeplant parts (Pharr et al., 1986). The effects of fruits on gas exchange properties and carbon allocation in drought stressed cucumber plants have not been studied. Osmotic adjustment has been reported to increase plant tolerance to drought stress by enabling the plant to maintain cell turgor and tissue hydration at lower water potentials . A number of plant species have been shown to undergo osmoregulation in response to water deficits but it has not been demonstrated to occur in cucurbits. Since cucumbers originated in the semi arid regions of Africa and southwest Asia, drought tolerance or avoidance genes would be expected to be found within a diverse population of Cucumis sativus. Limited research has been conducted on the responses of pickling cucumbers to drought stress. This study was conducted with the following objectives: (1) to identify genotypic differences in responses to drought that might exist among selected cucumber parental lines and cultivars: (2) to study the effects of water deficits on gas exchange characteristics of cucumber leaves; (3) to evaluate the osmotic adjustment capacity of cucumber leaves in response to drought stress and (4) to identify the effects of fruiting on carbon assimilation and allocation in drought stressed cucumber plants. Literature Review Water deficits have adverse effects on plant growth and development. Leaf and stem growth is often retarded and reproductive organs frequently abort under drought stress conditions (Kramer, 1976). Plants have evolved several mechanisms to avoid or withstand drought stress. Thicker cuticles, leaf rolling, stomatal closure and development of extensive root systems are some of the water conservation mechanisms utilized by plants (Simpson, 1980; Turner and Kramer, 1980). Plant responses to water stress have been extensively covered in a number of review articles and books (é.g. Hsiao, 1973; Kozlowski, 1966-1980, Kramer,1983; Levitt, 1980; Turner and Kramer, 1980). Effects pp Plant Growth. peveloppent and Yield Effects pp vegetative growth Water deficits have direct and indirect adverse effects on plant growth. Direct effects include those on cell division and cell enlargement. Cell division and enlargement are equally sensitive to water stress (Meyer and Boyer, 1972; McCree and Davis, 1974). Leaf elongation becomes slower and eventually stops as soil water tension increases (Acevedo et al., 1971). Retardation of leaf 4 expansion at low water potentials was run: due to lack of substrates (Michelena and Boyer, 1982), however, formation of new leaf primordia is more sensitive to a limited supply of assimilates than is leaf expansion (Milthorpe, 1959). Overall plant growth is reduced as a result of water stress. Cucumber vine and leaf growth are reduced by water deficits (Cummins and Kretchman, 1975). Water stressed cucumber plants have fewer nodes and smaller vines (Ortega and Kretchman, 1982). Plant growth is also indirectly affected by drought. Decreases in nutrient uptake, particularly phosphorus, are observed in water stressed plants (Ackerson, 1985). Plant hormone levels are altered in stressed plants. Abscisic acid (ABA) induces stomatal closure resulting in decreases in the production of assimilates needed for growth. ABA levels increase in water stressed plants (Eze et al., 1983; Raschke et al., 1976). The effects of water stress on photosynthesis will be dealt with in more detail in another section. Effects pp fruit set and fruit gpplipy Economic yield of a pdckling cucumber crop is dependent on the number, weight and quality of fruits produced. Drought..adverse1y’ affects. pollen. gualityu In squash (Cucurbita pppp L.) and Phaseolus vulqaris L., dehydration reduced. the ,pollen :germination jpercentage, resulting in reduced fruit set and number of seeds per fruit (Gay et al., 1987; Shen and Webster, 1986). A decrease in pollen 5 viability in water stressed cucumbers might account, in part, for yield reductions. Doss et a1. (1977) found that pickling cucumber yields were decreased when more than 70% of the available soil moisture was depleted. Subjecting bush bean plants to soil water tensions of 0.75 bars or more reduced yields by 48% (Stansell and Smittle, 1980). Water stress during the flowering stage caused the largest decrease in bush bean yields (Dubetz and Mahalle, 1969). Cucumber fruit quality' is also affected. by ‘water deficits. Cucumber fruits developing under conditions of water stress would have a poor quality, mainly due to the increase in the incidence of carpel separation placental hollows, and fruit deformation. Elkner (1982) reported that plants growing at a soil water tension of 0.45 bars produced ‘57.5% of their fruits with either carpel separation or placental hollows. The decrease in cucumber fruit quality is apparently due to a decrease in photosynthate production. Kanahama and Saito (1985a) found that defoliation and leaf shading of cucumber plants increases the incidence of crooked fruits; fruit curvature increased as the leaf area/fruit decreases. The results of another study by Kanahama and Saito (1985 b) suggest that competition for available assimilates increases the incidence and degree of fruit curvature. Water deficits reduce photo—assimilate production and consequently, would be expected to have a similar effect on fruit shape. 6 prpp Regpirements p: Cucumbers Crop water requirements are highly dependent on environmental factors such as air temperature and relative humidity, wind velocity and sunlight intensity and duration. Consequently, estimates of the water requirements for a cucumber crop vary with the conditions under which measurements were made. Reported values vary between 3.5- 5.5 mm/day (Loomis and Crandall, 1977) to 8 mm/day (Ritter et al., 1984). strategies pp; Dealing Kipp flpppp Deficits Cultural practices A number of management practices have been employed in an effort to avoid or delay plant exposure to drought stress. Some of these practices are useful only in arid and semi— arid climates while other practices may also be beneficial in temperate climates. Examples of commonly used practices include: - soil management and irrigation Fallowing, to increase stored soil water, is frequently utilized in semi-arid locations to delay the onset of water deficits (French, 1978). In some soils, crusting can occur under drought conditions resulting in poor germination and stand establishment. Sowing germinated seeds in a fluid gel is a technique that is useful where soil crusting can occur (Taylor et al., 1982). However, a prolonged period of drought following sowing would be detrimental to the germinated seeds. The use of irrigation is dependent on 7 economic considerations and, particularly in arid regions, on the availability of an adequate water supply. - antitranspirants Antitranspirants, compounds that reduce plant transpiration, have been tested for use in reducing plant water stress, but are not extensively used commercially. wax: emulsions, polyvinyl chloride and ikaolinite are examples of antitranspirants that act as physical barriers to transpiration by forming an impermeable film on the leaves. Phenylmercuric acetate and hydoxyquinoline sulfate are {antitranspirants that. induce stomatal closure, thus reducing transpiration. All antitranspirants reduce C02 entry into leaves and consequently decreases in photosynthesis and yield are often observed. Bravdo (1972) and Davenport et al.(1974) reported decreases in photosynthesis, plant growth and yield following the application of antitranspirants. In contrast, Rao (1985) obtained significant increases in tomato yields following the use of antitranspirants. However, these yield increases were due to increased fruit water content as more water became available upon reducing transpiration. Abscisic acid, applied as an anti-transpirant has been found to improve seedling survival following transplanting, and to increase plant water potential and fruit yield (Berkowitz and Rabin, 1988). - early cultivars Planting cUltivars that mature before the onset of severe drought stress is a useful practice in regions where the beginning of the dry season is clearly defined. In Australia, higher grain yields were obtained in early maturing spring wheat cultivars as compared to late maturing cultivars (Fischer and Maurer, 1987; Reitz, 1974). Plant breeding Breeding for drought tolerance is a long term approach for dealing with water deficits. Several morphological and physiological traits, such as root depth, stomatal frequency and sensitivity and the capacity for osmotic adjustment, are associated with drought tolerance in a number of plant species. Genotypic differences in these traits can potentially be used to increase drought tolerance in crops. Varietal differences in root growth patterns have been reported in tomatoes (Gulmon and Turner, 1978), soybeans (Raper and Barber, 1970) and wheat. Rooting depth is a heritable trait that can be selected for by breeding (Hurd,1974) . Stomata that are sensitive to changes in soil moisture would allow plants to conserve their water and delay the onset of water deficits. Significant differences in stomatal sensitivity of different sorghum genotypes were reported by Henzell et al.(1976). A decrease in stomatal frequency might decrease transpiration. Miskin et al. (1972) reported that stomatal frequency is a 9 heritable trait in barley, and that a decrease in number of stomata reduced transpiration but not photosynthesis. Osmotic adjustment is a drought tolerance mechanism that is potentially advantageous to crops that are exposed to intermittent periods of water deficits. Differences in the osmoregulation capacity of sorghum and wheat genotypes have been reported (Ackerson et al., 1980; Fisher and Sanchez, 1979; Morgan, 1977; Stout and Simpson, 1978). Genotypic differences in drought tolerance of wheat cultivars have been attributed to differences in their capacity to osmotically adjust (Blum et al., 1983; Keim and Kronstad, 1981; Morgan, 1977), and variation in osmoregulation was positively correlated with grain yield (Morgan et al.,1986). Osmoregulation. is a: heritable trait ‘that is controlled by a single gene (Morgan, 1984). Morgan et al.(1986) suggested using this characteristic in screening for drought tolerant wheat lines. However, differences in drought tolerance may not reflect differences in osmoregulation. Jones and Turner (1978) did not find significant differences in osmoregulation between two sorghum cultivars that differed in drought tolerance. Plant Adaptations pp Water Deficitp: A number of morphological and physiological traits have been associated with drought tolerance in plants. The most common adaptations include the following: 10 SW1 W Stomatal closure in response to decreasing soil moisture is a physiological adaptation to drought. Stomatal closure during the time of day when evaporative demand is high would conserve plant moisture and delay the onset of water deficit. Stomata of several species, e.g. apricot and sorghum, have been shown to respond to air relative humidity, closing as relative humidity decreases (Farquhar, 1978; Schulze aand, Kuppers,1979). Stomatal opening' when humidity is high would allow for photosynthesis to proceed with minimal water loss, thus improving the plant’s water use efficiency. Root growth: Changes in plant morphology have also been associated with development under drought conditions. One of the most common examples of morphological adaptations is the possession of a deep root system. An extensive, deep root system would allow the plant to extract water from a larger soil volume. Deep rooted plants, such as tomato and alfalfa, are thus able to delay the onset of water stress. Genotypic differences in drought tolerance of some wheat varieties are due to differences in rooting depth (Hurd ,1974). Stressed plants allocate more dry matter to roots at the expense of shoots resulting in a larger root to shoot ratio(Huck. et al.,1983); this potentially reduces transpiration and increases water uptake. 11 ___Leaf refine: Leaf rolling is a mechanism that might have an adaptive value in drought tolerance. A decrease in light interception and consequently, a decrease in leaf temperature would be advantageous under water-l imiting conditions. Wudiri and Henderson (1985) reported that the tomato cultivar ’saladette' rolled its leaves in response to water stress and suffered a 40% reduction in fruit set, while another cultivar ’VF 145b-7879’ that did not roll its leaves, suffered a 90% reduction in fruit set. Osmotic adjpstmppp: Osmotic adjustment is suggested as a process by which plants can become more tolerant. of low soil ‘water potentials (Morgan, 1977; Turner and Jones,1980). This response to water stress will be discussed in more detail in the following section. Osmotic Adjustment ip Plants Role 1 dropght tolerance Osmotic adjustment is one of the mechanisms that plants have developed to avoid tissue dehydration under water-— limiting conditions. Osmotic adjustment may be described as the decrease in cell osmotic potential caused by the active accumulation of solutes in response to water or salt stress. A decrease in osmotic potential resulting from cell dehydration is not considered an osmotic adjustment. Turner and Jones (1980) differentiate between the terms osmotic adjustment and osmoregulation which are frequently 12 used to refer to the same process. They suggested using the first term when referring to this process in higher plants and the latter for microorganisms. In this review, both terms will be used interchangeably. A plant's water status may be defined by its water potential, which is equal to the sum of the osmotic (solute), pressure, gravimetric and matric potentials. For cell expansion and many other physiological processes to proceed, the pressure potential has to be positive. The threshold cell turgor pressure for growth to occur varies with species, environmental and other factors. While osmotic and water potentials always have negative values, the possibility of a negative pressure potential occurring in cells was disputed by Tyree (1976) who attributed the reported negative values to errors in measuring osmotic potential. Osmotic adjustment has a role in plant tolerance to water stress through maintaining positive cell turgor. This is achieved via a decrease in osmotic potential in response to water deficit (Morgan,1977; Turner and kramer,1980). Such a process would allow for continued root growth and maintenance of stomatal opening (Graecen and Oh,1972; Van Volkenberg,1985). A number of plant species have been shown to undergo osmotic adjustment in response to water stress; included are tomato, pea, bean, apple, sorghum, sunflower and wheat (Acevedo et al., 1979; Fanjul and Rosher, 1984). Plants that osmotically adjust are capable of maintaining 13 leaf turgor to lower water potentials than those that do not (Ackerson, 1981; Ackerson and Hebert, 1981). At low water potentials, pressure potentials and water content of adjusted plants are higher than those of non-adjusted plants, reflecting the role of osmotic adjustment in maintaining tissue hydration and thus, survival under stress conditions (Flower and Ludlow, 1986; O’Neill, 1984). However, adjusted and non-adjusted plants reach zero turgor at the same relative water content. Studies indicate that osmotic adjustment is 21 rate dependent process. Slow rates of stress imposition were found to allow for more solute accumulation than rapidly developing stress (Flower and ludlow,1986;Thomas,1986). Strawberry plants were subjected to a rapid rate of stress of 1.2 Mpa per day; this rate did not allow for osmotic adjustment to occur while rates of 0.15 and 0.7 Mpa per day allowed for equal levels of adjustment (Jones and Rawson,1979). Osmoregulation 1J1 fruits has run: been extensively studied. Fruits. of stressed. cucumber' plants were reported to have a higher concentration of solutes than fruits of non—stressed plants (Ortega and Kretchman,1982). However, it in”; unclear whether the increase was due to an increase. in solute content or to dehydration. Solutes involved :U1 osmoregulation apparently' become available for plant metabolism following relief of stress. Consequently, osmotic adjustment is maintained for varying 14 periods of time following rewatering depending on rate, severity and duration of water stress. Following one stress cycle, the osmotic potential of cotton leaves returned to pre-stress levels within six days of rewatering while plants subjected to several stress cycles maintained low solute potentials for up terUD days after rewatering (Oosterhuis and Wullschleger, 1987, Shahan et al.,1979). The degree of osmotic adjustment also varies with species. Osmotic potential at full turgor decreased by 0.1 to 0.4 MPa in maize, sorghum and sunflower plants subjected to water stress (Sanchez-Diaz and Kramer,1978). Solutes ip osmotic adjustment: A number of solutes have been associated with osmoregulation. Sugars, organic acids, potassium and chloride ions, proline and betaine are some of the most commonly reported osmotica. Glucose is the main solute that accumulated in leaves of stressed cotton plants (Ackerson,1981) while non-reducing sugars were reported to accumulate in stressed sorghum (Acevedo et al.,1979). Tomato cell cultures subjected to low water potentials underwent osmoregulation with reducing sugars accounting for only 20 % of the decrease in osmotic potential; potassium, chloride and amino acids accounted for the remaining 80 percent (Handa et al.,1984). Proline is another solute that has been associated with plant responses to water deficits. The level of proline in leaves acts as an indicator of stress, but its accumulation does 15 not reflect drought tolerance (Blum and Sullivan, 1974). Betaine levels have also been reported to increase in water stressed plants. Proline and betaine might have a protective role for enzymes in stressed tissues (McCree,1986). Advantages and limitations: Turner and Kramer (1980) suggested that osmotic adjustment has the following advantages: a- maintenance of cell turgor and elongation. b- maintenance of stomatal opening and photosynthesis. c- allow for continued root growth. Osmotic adjustment 1J1 roots provides an1 additional advantage that is the maintenance of water uptake at lower soil water potentials. The benefit from root osmotic adjustment is limited by environmental conditions such as soil type and evapotranspirative conditions. A light soil has a lower water holding capacity than a heavy soil. Consequently, for a plant growing in a light soil, a smaller increase in available water would be expected per unit of root osmoregulation. Some of the limitations that were cited by Turner and Kramer include the loss of adjustment within a few days of relief of the stress and the limited range of plant water potentials within which turgor can be maintained through osmoregulation. It can be concluded that osmotic adjustment would allow plants to tolerate short term water deficits, as sometimes occurs during the growing season in a —¥—‘ l6 temperate climate. Effects p;_ Water Deficits pp Plant Gas Exchange Characteristics Stomatal responses Environmental factors have direct effects on gas exchange characteristics of plants. Stomatal conductance is influenced by soil water potential and air humidity. Stomatal closure in response to decreases in humidity has been attributed to a direct effect of humidity on stomata that is independent of the leaf water status (Schulze and Kuppers,1979;Schulze and Hall,1982,). Stomatal responses to humidity ,not involving changes in leaf water status, are controlled by turgor of the epidermis and are referred to as feed-forward control (Farquhar,1978). Changes in stomatal conductance in response to changes in leaf water status occur through feedback control (Cowan,1977, Farquhar,1978). Several studies have indicated that a relationship exists between leaf water status and stomatal conductance. Stomatal closure was reported to occur at a threshold value of leaf water potential that varied with several factors including species, leaf age and stress history ( Ackerson, 1980; Sionit and Kramer, 1976). More recent studies have demonstrated that stomatal responses to mild soil water deficits were independent of leaf water potential and l7 turgor pressure. Blackman and Davies (1985) divided the roots of maize seedlings between two pots such that one was watered and the other was allowed to dry. This resulted in partial stomatal closure although. leaf"water' potential, turgor potential and abscisic acid levels were unaffected. In a different approach, Gollan et al. (1986) maintained leaf turgor in stressed plants by placing the root system in a pressure chamber; the stomata still closed irrespective of leaf water status. It can be concluded that stomatal conductance is directly affected by soil water status, independent of leaf turgor. Gollan et al.(1986) and others (Bates and Hall,1982;Bennett et al.,1987; Blackman and Davies,1985), suggested a role for cytokinins in root to shoot communication with a continuous supply of the hormones from the roots being required for complete stomatal opening. Osmotic adjustment, leading to turgor maintenance, allows plants to maintain stomatal opening under conditions of water stress. Repeated exposure to water deficits induced osmoregulation in sorghum, cotton and sunflower; this allowed plants to maintain higher stomatal conductances at lower water potentials, as compared to non-adjusted plants (Ackerson, 1980; Jones and Rawson,1979). Photosynthesis: Plants generally respond to decreases in available soil moisture by stomatal closure which is thought to be a major cause for' the observed decline in photosynthesis Pr fa ir 18 (Raschke and Hedrich, 1985). Other causes of the decline in Pn rate has not been clearly identified, but a number of factors have been suggested as causes of the decline; included are: the accumulation of assimilates (Ackerson,1981), localized low water potentials at evaporation sites in the mesophyll (Sharkey,1984) and reduced photochemical activity (Boyer,1971). Downton et al.(1988) concluded that stomatal closure leading to decreased intercellular C02 levels can fully account for the observed decline in photosynthesis in water stressed plants. A similar conclusion was reached by Raschke and Hedrich (1985). Other studies have indicated that the decrease in photosynthesis in water stressed plants is not solely due to stomatal closure, as mesophyll conductance was also found to decrease; this was suggested to be due to the accumulation of assimilates (Ackerson and Hebert, 1981; Thorne and Koller,1974). Bunce (1982) did not find a correlation between mesophyll conductance and total non-structural carbohydrates content of stressed leaves; the increase in carbohydrate content did not account for the decline in Pn rate. Direct inhibition of photosynthesis by water stress has been attributed to a decrease in choloroplast volume, leading to increases in concentrations of inhibitory solutes such as K+ (Kaiser,l986). However, Sharkey and Badger (1982) disputed the possibility of such an effect. Others have reported that stress has direct effects on chloroplasts which lead 19 to the observed reductions in photosynthesis (Genty et al.,1987). A similar conclusion was reached by Krieg and Hutmacher (1986) who found that assimilation rate was lower at all internal C02 levels in water-stressed plants. Berkowitz and. Gibbs (1983 a,b) concluded. that photosynthesis was inhibited at low osmotic potentials due to stromal acidification which inhibited the activity of the jFructose 1,6-biphosphatase. Later, Pier' and Berkowitz(1987) found that K+ has a protective role involving the exchange of cytoplasmic K+ for H+ in stroma, which restored stromal alkalization and photosynthetic activity. Disruption of chloroplast thylakoid membranes has been observed in leaves of stressed plants; this may be a cause for the observed decrease in photosynthesis in stressed plants (Johnson et.al, 1982). Photosynthesis might acclimate to low water potential, thus allowing for CO2 fixation to continue under water stress conditions (Matthews and Boyer, 1984). Osmotic adjustment has a protective role for the photosynthetic apparatus (Downton,1983), allowing' photosynthesis to continue under stress conditions until turgor is lost (Boyer and Potter,1973). Water deficits also affect overall plant photosynthesis by limiting leaf growth and thus reducing the potential photosynthetic capacity of plants (Acevedo et al., 1971). 20 Water use efficiency: Water use efficiency (WUE) may be defined as the ratio of carbon dioxide uptake to water transpired by a plant. The definition may be generalized and expressed as the ratio of dry matter produced to evapo- transpiration of a crop. For a plant growing on a limited supply of soil water, water use efficiency is important in determining the potential of that plant for growth and yield. A high plant WUE reflects more growth per unit of available water, as compared to plants with low WUE. Water use efficiency is influenced by a number of plant and environmental factors. Vapor pressure deficit, a function of leaf and air temperature and relative humidity, influences stomatal conductance and transpiration and consequently water use efficiency of a plant. In cassava, water use efficiency decreased as vapor pressure deficit increased ( Cock et al., 1985), and no difference in WUE between stressed and non-stressed was observed (El- Sharkawy and Cock,1984). Jones (1976) reported that WUE increases as stomatal resistance increases and as boundary layer resistance decreases. Similarly, daily WUE increased when plants avoided inn; peak transpiration period by closing their stomata in response to increased vapor pressure deficit at mid-day (Ludlow, 1980). Nobel (1980) developed. a theoretical basis for a relationship between cell size and water use efficiency. He attributed the higher WUE values observed in plants that 21 develop under conditions of water stress, to the smaller size of cells produced under these conditions, as compared to non—stressed conditions. Leaves developing under conditions of high temperature, high irradiance and soil salinity would be expected to have small cells and high WUE values. Translocation: Water stress apparently has no direct effect on translocation and phloem loading. The observed decrease in translocation rates in water stressed plants is probably due to a decrease in assimilate production as photosynthesis declines. Sung and Kreig (1979) found that CO2 assimilation is more sensitive to water deficits than is translocation. Contrary to that, Brevedan and Hodges (1978) reported that translocation is more sensitive to water deficit than photosynthesis. Effects p; Fruiting pp Photopynthepip App Assimilate pllppppion Actively growing‘ fruits act as sinks for photo- assimilates. Assimilate demand influences photosynthesis and translocation. Increases in photosynthesis and in carbohydrate export from source leaves is observed when the source to sink ratio is decreased (Thorne and Keller, 1974). Net photosynthetic rates have been reported to be higher in fruiting than in non-fruiting plants of several species. 22 Carbon exchange, assimilate export and starch accumulation rates are higher in fruiting than in vegetative cucumbers; the increase in Pn rate was associated with increased sink demand (Pharr et al.1985). Net photosynthetic rate (Pn) is higher in fruiting than in de-blossomed pepper plants (Hall,1977). A similar observation was made on strawberry; however, on a whole plant basis, net photosynthesis was similar in fruiting and non-fruiting plants due to the larger leaf area in non- fruiting plants (Choma et al., 1982). DeJong (1986), concluded that increased photosynthetic rates in fruiting Prunus persica trees were mainly due to increases in leaf rather than mesophyll conductance. Fruit and flower removal have been associated with decreases; in photosynthetic rates. King et al.(1967) observed a 50% decrease in Pn rate of the flag leaf within hours of ear removal in wheat. The photosynthetic rate regained its previous level when other leaves on the plant were darkened, thus precluding them as sources for the young shoots and roots. The cause of the observed decreases in Pn rate upon flower or fruit removal is not clearly identified. Some have attributed the decline in Pn rate to increased stomatal resistance (Gifford and Marshall,1973;Rawson et al.,1976) and to increased leaf and meSOphyll resistance (Hall enui Milthorpe, 1978). Mesophyll and stomatal conductance of fruiting strawberry plants were 40% higher than those of de-blossomed plants E 23 (Forney and Breen,l985). These results imply that the decrease in Pn rate is due to a limited CO2 availability. This is in contrast with the findings of Crafts-Brander (1987) who reported that in some maize genotypes, ear removal resulted in a decrease in Pn rate which was not due to limited C02 availability as the internal C02 concentration increased upon ear removal. Accumulation of assimilates in chloroplasts of source leaves has also been suggested as a cause for the decrease in Pn rate upon defruiting (Choma et al.,1982). Leaf starch concentration was negatively correlated with photosynthetic rate in soybean (Nafziger and Koller,1976). Disruption of choloroplast ultra- structure as a :result of excessive starch accumulation is a possible cause for the decrease in Pn rate (Schaffer et al.,1986); however, this is difficult to reconcile with the rapid recovery in Pn rate (King et al.,1967). Fruit bearing alters the dry matter partitioning strategy of a plant. Developing fruits represent strong sinks which actively compete for the available assimilates. Fruit growth retards shoot and root growth in cucumber, reflecting the strength of fruits as sinks to which assimilates are preferentially allocated. An estimated 40% of the total amount of photo-assimilates produced by the plant are required for the growth of a single fruit (Pharr et al., 1985). Barrett and Amling (1978) found that within 24 hrs of their production, 80% of assimilates were translocated to the fruit. This might be a 24 reason for the limited number of fruits that can develop simultaneously on a cucumber vine. In Capsicum annuum, 90% of the assimilates produced are deposited in the fruit. Upon defruiting, partitioning of dry matter among the vegetative parts becomes evenly balanced (Hall,1977). Loomis and Crandall (1977) observed that fruiting cucumber plants had 21% less total leaf area than defruited plants. Similar observations were made on strawberry (Choma et al., 1982; Schaffer' et al., 1986). Fruiting strawberry plants had 62% and 44% less dry matter in roots and leaf blades, respectively, than de-blossomed plants (Forney and Breen,l985). The inhibitory effect of fruits on vegetative growth have also been attributed to inhibitors exported by developing' fruits (Barrett. and. Amling, 1978). The final total dry weight of reproductive and vegetative parts are equal in fruiting and deflowered plants (Choma et al., 1982). The higher net photosynthetic rate apparently compensates for the smaller leaf area of fruiting plants and allows for the production of an equal amount of dry matter (Schaffer et al., 1985). Growing cucumber fruits can also inhibit the growth of other fruits on the same vine (McCollum, 1934). Ells (1983) found that in pickling cucumbers, the inhibitory effeCt of pre—existing growing fruit did not extend beyond 10 nodes from an existing fruit. Effects of water deficits and fruiting on carbon assimilation and allocation in pickling cucumber plants Abstract Gas exchange measurements were made on leaves of Cucumis sativus L. plants subjected to drought stress. Plant water potentials were allowed to decrease to < -0.7 Mpa, during the flowering and fruiting growth stages. Assimilation rates (A) were measured at saturating PPFDs, for non- stressed plants, 1000 umol.s"1m"2 or higher. Leaf temperatures during these measurements ranged from 22C to 32C which were found not to affect A. Drought stressed plants had 63% to 73% lower CO2 assimilation rates than well watered plants. Stomatal conductances ranged from 0.13 to 0.14 cm.s"l, 80% lower than gS of leaves of control plants. The adverse effects of water deficits on photosynthesis were reVersible. Within 12 hours after rewatering, CO2 assimilation rates of previously stressed plants increased. to 11.7 umol.s'lm-2, not significantly different from that of irrigated control plants. The decrease in intercellular CO2 levels accounted for 36.5% of the decrease in A, while the remaining 63.5% of the decrease was attributed to non-stomatal factors. Water use efficiency (WUE) decreased rapidly as leaf-air VPD increased above 2 Kpa. Drought stressed plants tended to have higher WUE than control plants. In drought 25 26 stressed and non stressed plants, CO2 assimilation rates of fruiting' plants ‘were ihigher than those of non-fruiting plants. Under both water regimens, fruiting plants allocated assimilates to developing fruits at the expense of leaves, stems and roots. It is concluded that the effects of water deficits and fruiting on photosynthesis cannot be explained solely by observed changes in stomatal conductance. Introduction Cucumbers are fleshy plants which have a high water requirement for growth and development(26,32). In the mid- western United States, periods of drought, 7 to 10 days in duration, are common during the summer months, June through August (34), and lead to moderate to severe water deficits :hi rain—fed cucumber crops. Transient water deficits are also frequently observed in cucumber plants due to high transpirative ‘water loss at :mid. day. Such water deficits result in temporary leaf wilting and stomatal closure enui ultimately' in a reduction in photosynthesis (2,14). Cucumber fruit yield and quality have been reported to decrease under conditions of drought stress (8,10). Decreases in cucumber fruit quality have been associated with the decrease in photoassimilate supply (22) which can be expected under conditions of drought stress. Plant water status and fruit set have been shown to influence photosynthetic activity 27 (2,14,18,21,24,28). The cause, of the decrease in photosynthesis in water stressed plants is still run: completely understood. Under water deficit conditions, CO2 fixation rates are low due to decreased intercellular CO2 levels (30), accumulation (If assimilates (IJ and/or localized low water potentials in the mesophyll (38). Fruiting plants have higher photosynthetic rates than defruited or vegetative plants (18,28). Increased photosynthetic rates have been attributed to higher stomatal conductances (7,31) and higher mesophyll conductances (13,18) in fruiting as compared to non- fruiting plants. Water deficits and fruiting also impact upon dry matter partitioning in plants. Water stressed plants tend to allocate more photoassimilates for root growth at the expense of leaf and stem growth (20,25) which ultimately reduces the photosynthetic leaf area of the plant. In a similar manner, cucumber fruits can limit leaf growth and development by competing with vegetative parts and other fruits for the available photoassimilates (3,11,28) due to their strong sink strength. The combined effects of environmental and plants factors on dry matter production capacity, and consequently potential productivity, and dry matter allocation in cucumber plants have not been studied. An understanding of these factors is needed before a strategy can be developed for improving the crop’s performance under water-limiting conditions. This study 28 was undertaken: (1) to determine the effects of water deficits, light, temperature and vapor pressure deficit on the leaf gas exchange properties in cucumbers, (2) to evaluate the recovery of photosynthetic activity following relief of water stress and (3) to investigate the combined effects of water deficits and fruiting on dry matter production and partitioning in cucumber plants. Materials and Methods Plant material: In greenhouse experiments, seeds of the pickling cucumber (Cucumis sativus L.) inbred lines GY14 and M21 were sown in a 1:1 peat (Baccto professional mix) sandy loam soil mixture in 11-liter plastic containers. Plants were fertilized twice weekly using Peter’s 20N-8.8P- 16.6K soluble fertilizer at a concentration of 0.2 g.l-l. Pistillate flowers were hand-pollinated between 10 inn and 12 noon on the day the flowers opened. Day/ night temperatures were maintained at about 30 / 20C +~- 5C and no supplemental lighting was provided. Cucumber plants were also cultured in a field environment during June through August, 1987, by planting seeds into 11-liter plastic containers buried in the soil at the Horticulture Research Center of Michigan State University. Two irrigations during the vegetative stage supplemented natural rainfall. When all plants were. bearing fruits, plants were transferred to the greenhouse for additional 29 measurements. Water regimen: Water deficit treatments were induced by withholding water from the plants for 3-4 days until the plants were visibly wilted throughout the day and the dawn-plant water potential had reached -0.6 in: —0.8 Mpa. Stressed plants used in studying recovery of photosynthetic activity were rewatered 12 hours before gas exchange measurements were made. Control plants were watered daily. Deflowering. Fruit set and development were prevented by removal of pistillate flowers from the plants on a daily basis throughout the experiment. Leaf gas exchangp measuremenpgp Gas exchange responses to light, temperature and CO2 concentration of the 4th or 5th attached leaf from the shoot apex were determined using an open gas exchange system previously described by Sams and Flore (37). Each leaf was enclosed in a 20 cm x 10 cm controlled environment chamber. Leaves were allowed to equilibrate with the micro—environment of the chamber for a period of 2 hours before gas exchange measurements were made. To determine the light response curve, gas exchange measurements were made at several levels of PPFD beginning at a flux density of 1800 umol.s"lm"2 and incrementally decreasing to total darkness. Ambient CO2 temperature were maintained constant at 345 +—5 ppm and temperature of 25 +-0.5C, respectively. 30 The temperature response curve was determined by raising leaf temperature from an initial temperature of 10—15C up to 40C in increments of BC to SC. Vapor pressure deficit was maintained below 1.5 kPa up to a temperature of 30C above which VPD increased rapidly. The CO2 responses of leaves of differentially watered plants was determined by exposing the leaves tx> ambient C02 levels of 150 to 350 ppm. Determinations of net CO2 assimilation rate (A), photosynthetically active radiation, relative humidity and leaf temperature, under greenhouse and field conditions, were made using a portable open system LCA-Z (Analytical Development Corporation, Hodesdon, England) infrared CO2 analyzer operated in differential mode, an air supply unit at a flow rate of 600 cm3.min"l, and a Parkinson broadleaf leaf chamber with a window area of 6.25 cmz. Stomatal conductance (gs), transpiration rate (E) and vapor pressure deficit (VPD) were calculated using computer programs developed by Moon and Flore (28). All measurements, except for' the. diurnal measurements, were made under sunlight between 10:30 A.M. and 12:30 P.M. Ambient C02 levels were between 325 and 348 ppm. Measurements were made on the fourth and sixth leaf from the shoot apex of each plant. Treatments were replicated 3 times in a randomized complete block design with 2 plants per treatment in a replicate. Measurements of diurnal changes in gas exchange characteristics were made at 10 A.M., 2 P.M. and 6 P.M., 31 under a HID low pressure sodium lamp such that the measured PAR at the leaf surface was always greater than a saturating level of 1000 umol.s'1m'2. Gas exchange measurements under field conditions were made on July 30th which was a clear, sunny day. Leaf sugar determinations. Leaf samples were freeze dried for 24 hours then finely ground with a mortar and pastel. Sugars were extracted from tissue subsamples (0.2 g) with 80% ethanol at 70C for 1 hour. The extract was filtered through a No. 1 Whatman filter paper and the ethanol evaporated. The residue was re—dissolved in 25 nfl.«of deionized water an an aliquot of the resultant solution filtered througha 0.45 um Millex-HA filter unit. Sugars and sugar alcohols were assayed using a Dionex Carbopac PA1 anion exchange separation column with a Dionex series 40001 High Performance Ion Chromatography Module and a pulsed amperometric detector with a gold electrode. A 0.1 M NaOH solution was used as the eluant. Results Carbon dioxide assimilation rate reached saturation at approximately 900 umol.m-2s”li (Fig. 1). Subsequent measurements of assimilation rates in the field, greenhouse and laboratory were conducted at PPFD levels higher than 1000 umol.m-23'1' to assure light saturating' conditions. Maximum carbon dioxide assimilation rates were measured 32 at stomatal conductances greater than 0.4 cm.s-l while the transpiration rate continued to increase until gs reached 1 0.6 cm.s' (Fig 2). Temperature also influenced CO2 assimilation below 16C and above 34C (Fig 3). Within the range of 16 to 34C , assimilation rates did not fluctuate significantly. High temperature, greater than. 34C, resulted. in 21 decline 1J1 assimilation :rate concomitant with an increase in VPD. Subsequent gas exchange measurements in the field and greenhouse were made at ambient temperatures of 22 to 32C. Water use efficiency decreased rapidly as vapor pressure deficit increased above 1 kPa (Fig 4), but stabilized at a low level of WUE at VPD of 2 kPa or higher. Drought stressed greenhouse and field plants had 63% and 73% lowerico2 assimilation rates than well watered plants (Table 1). Stomatal conductances of drought stressed plants averaged 0.13 to 0.14 cm.s"1, which is about 80% lower than gs in control plants. Plant water potentials recovered rapidly and completely within 12 hours after rewatering (Table 2). Zni drought stressed plants, water potentials increased from -0.77 MPa to a potential not significantly different from non- stressed plants, -0.1 MPa. The osmotic potentials of stressed plants were lower than those of control plants, indicating that leaves of drought stressed plants had undergone osmotic adjustment in response to the stress. 33 .BcoEoSmooE 6:229: Ecomotaoc Ban 300 .>_o>:ooamoc .Eae 0mm. pco 0mm Lo poEBEoE 8oz, cozobcoocoo N00 pco ochoLanoL ecoEE< .BcoE cocEsozo Lo xodo Loocm 9: E0: u6o. 5m 9: Lo 30.. ozoficxmouocd :0 Quad mEmovi co Bootm é .911 ch ...E .683 :3: code 00.: 00.2 00.? oo_: 0mm can own own. 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Plants were grown in plastic containers in the field then transferred to the greenhouse 1 week before measurements were made. y. Watering was withheld from the plants until plant water potential decreased to 0.5 to 0.8 Mpa. 38 Table 2. Water use efficiency and water status of green- house grown cucumber plants following gas exchange measurements. 2 WUE Potential (MPa) (mmol CO / Treatment mol H20 Water Osmotic Pressure Drought stressed 4.3 -0.77 -0.82 0.05 Y Drought stressed/ rewatered 5.1 -0.10 -0.71 0.61 Non-stressed 4.1 -0.08 -0.63 0.55 L.S.D (0.05) 0.08 0.05 0.06 z. WUE: Water use efficiency. y. Plants were subjected to water deficit then rewatered 12 hours prior to time of measurement. 39 Recovery of plants from a water deficit condition following rewatering was rapid for cucumbers. C02 assimilation rates increased from 3.5 to 11.7 umol.s"]'m"2 at 350 ppm C02 within only 12 hours after reWatering (Table 3). Increasing ambient CO2 concentration from 150 ppm to 350 ppm caused significant increases imllk and in the estimated intercellular CO2 (Ci) concentration. The Ci was calculated according to the model suggested by Downton et al.(1988). Comparison of the CO2 assimilation rates of stressed and control plants at similar C- theoretically allows one to evaluate the mode 1 by which water deficits have an inhibitory effect on photosynthesis. In water stressed plants exposed to 350 ppm ambient C02, Ci was estimated at 66 umol.mol’l with an A of 3.5 umol.s'1m’2. At a similar ci level (69.7) in a well watered plant (exposed to 150 ppm C02), assimilation rate was 9.6 umo1.s"1m'2 which is approximately 1.75 times higher than the rate (3.5 umol.m-2.5'l) measured in plants experiencing water deficit. Well watered fruiting plants had a 24.4% higher A than that of deflowered plants (Table 4). Under water limiting conditions, A in fruiting plants was 31% higher than A of deflowered plants. Fruiting plants had higher stomatal conductances as compared. to (deflowered. plants. Fruiting drought-stressed plants had the highest WUE (2.17) while deflowered and non—stressed plants had similar WUE that ranged between 1.53 and 1.60 mmol C02 per mol H20. 40 Table 3. Effects of water deficits and CO2 level on photosynthesis in cucumber leaves. A Water Ca (upol z y regimen (PPM) m s'l) Estimated Modelled Stressed 350 3.5+- 0.8 135.2+-l4 66.0 Rewatered 350 11.7+- 1.1 133.0+- 4 123.5 x Control 350 12.9+- 1.1 178.5+- 9 (178.5) Stressed 250 2.3+- 0.3 89.0+-14 50.6 Rewatered 250 11.0+— 1.2 92.1+-14 88.5 Control 250 11.2+- 1.2 114.2+- 6 (114.2) Stressed 150 1.5+- 0.4 75.8+-10 45.7 Rewatered 150 8.0+- 1.2 53.7+- 9 51.5 Control 150 -9.6+- 0.7 69.7+- 6 (69.7) 2. Intercellular CO levels were estimated according to y. Moon and Flore (1986). Intercellular CO level were calculated according to the model: C-=[(R-1)r+(C-,IRGA)]/R, as suggested by Downton et al. (1988), w ere C. is intercellular C02, R is the ratio of assimilation rate of control leaves to that of stressed leaves and r is the CO2 compensation point for photosynthesis. Cucumber leaves were assumed to have a C02 compensation point of 40 m. Igtercellular CO level in control plants is the same as that calculated from IRGA. 41 Table 4. Effects of water deficit and fruiting on gas exchange parameters of cucumber leaves. y z A X W.U.E Water (pgol gs (mmol C02/ Treatment stress m .s'l) (mm.s'l) mol H20) Fruiting w Yes 8.4 2.7 2.17 Deflowered Yes 6.4 2.1 1.52 Fruiting No 15.8 5.8 1.60 Deflowered No 12.7 4.7 1.53 L.S.D (0.05) Interaction 1.8 0.7 0.37 F— Significance Fruiting *** *** * Water stress *** ** NS Fruiting x Water stress NS NS NS * **, *** and NS. Significant at the 5%, 1% and 0.1% levels, and not significant, respectively. 2. Water use efficiency of individual leaves. y. CO assimilation rate of individual leaves. x. Leaf stomatal conductance. w. Plants were deflowered by removing pistillate flowers daily, throughout the experiment. 42 The highest leaf area per plant was produced on irrigated deflowered plants, 15410 cmz, which is 47.5% larger than the leaf area. produced. by non-stressed fruiting plants (Table 5). Under well watered conditions, fruiting plants produced 31.9%, 43.7%, and 38.6% less leaf, stem and root dry matter, respectively, as compared to non-fruiting plants. Drought stress had a major effect on fruit biomass per plant. Only approximately 20 g dry wt. of fruit was produced per plant under water stress conditions as compared to 66 g dry wt in irrigated plants. Total dry matter produced was similar in stressed fruiting and deflowered plants. Non stressed fruiting plants produced 23.8 c; more total dry matter than non-fruiting plants. Specific leaf weight ranged between 317 and 330 mg.dm"2 (Data not shown) and no fruiting and drought stress effects were found. The levels of translocate sugars in cucumber leaves were affected by water deficits and fruiting. The concentrations of sucrose and raffinose in leaves of stressed plants, 0.75 and 0.21 mg.g-l fresh wt., were more than double those detected in leaves of well-watered plants (Table 6). Stachyose concentration in leaves of drought stressed and deflowered plants ranged between 1.03 and 1.13 mg.g-1 fresh wt, significantly lower than the 1.65 mg.g'l fresh wt detected in leaves of well-watered fruiting plants. 43 Table 5. Effects of ‘water' deficit. and fruiting' on dry matter production and partitioning in cucumber plants. Z Y Leaf Dry weight (g.plant'l) Water are deficit (cm ) Leaves Stems Roots Fruits Fruiting yes 6250 20.6 23.4 2.8 19.6 Fruiting no 10445 33.3 25.8 10.7 66.2 Defruited yes 8575 28.9 31.4 3.8 ~- Defruited no 15410 48.9 45.8 17.5 -- x L.S.D (0.05) 1774 5.2 3.3 3.8 14.1 F-Siginificance Water *** *** *** *** ** Fruiting *** *** *** ** - Water x fruiting * NS ** * - 2. Leaf area measured at the end of the experiment, 51 days after planting. y. Dry weights, except fruit dry wt., were determined at the end of the experiment; fruits were multiple harvested for 3 weeks and dry weights determined upon harvest. x. L.S.D. for interaction except for leaf and fruit weight means where L.S.D is for main effects. **, *** and NS. Significant at the 5%, 1% and 0.1% probability levels, and not significant, respectively. 44 Table 6. Soluble sugar levels in the 5th leaf from the shoot apex of drought stressed and non-stressed fruiting and defruited cucumber plants. Concentration (mg.g-l fresh wt.) Reducing Treatment sugars Sucrose Raffinose 2 Water regimen Drought stressed 0.99 0.75 0.21 Well watered 0.99 0.33 0.10 L.S.D(0.05) NS 0.31 0.10 y Stachyose (mg.g'1 fresh wt) Treatment Fruiting,stressed 1.15 Fruiting,watered 1.65 Deflowered,stressed 1.13 Deflowered,watered 1.03 L.S.D (0.05) 0.15 2. Values are the averages of concentrations in fruiting and deflowered plants which were not statistically different. y. Plants were either allowed to set fruit or had all pistillate flowers removed and were either well watered or drought stressed. Discussion Light levels needed for CO2 assimilation rate to approach saturation. in.1greenhouse grown. plants were higher than those reported for growth chamber grown cucumber plants (35). Stomatal conductances lower than 0.4 cm.s"l apparently limited C02 availability and resulted in lower assimilation rates. As stomatal conductance increased, leaf transpiration rate continued to increase after A had plateaued. Transpiration has been reported to be a function of stomatal conductance and VPD (38). Temperatures in the range of 16 to 35C had no apparent effect on A in cucumber plants. The effects (ME higher ‘temperatures on leaf photosynthesis could not be elucidated because of the rapid increase in VPD which probably induced stomatal closure and consequently led to the observed decrease in A. In several plant species, stomatal opening is maintained at temperatures of up 36 degrees centigrade (21). Water use efficiency decreased rapidly as leaf-air vapor pressure deficit increased, which is in agreement with other reports (6). An increase in VPD would induce stomatal closure (38,39) 'which limits C02 availability and ultimately reduces photosynthesis (10,32). Concurrently, the increase in VPD leads to an increase in transpiration (43) which reduces water use efficiency. Comparison of the WUE to VPD relationship of greenhouse and field grown 45 46 cucumber plants indicates that at VPDs between 1.5 and 3.0 kPa field plants had higher WUE than greenhouse plants. Plants that develop under stress-inducing‘ conditions of high temperature, water deficits or high irradiance, which are characteristic of environmental conditions in the field, have smaller cells (44) and consequently higher WUE (29). The same reasoning can be used to explain the higher WUE observed in drought stressed greenhouse plants as compared to irrigated plants. The adverse effects of water deficits on photosynthesis were reversible. Recovery of photosynthetic activity following relief from drought stress was rapid. Sunflower plants are reported to have a threshold leaf water potential below which recovery of photosynthetic capacity following rewatering is incomplete (4). Incomplete recovery of photosynthesis has been reported to be due to incomplete stomatal opening (4). A decrease in stomatal conductance leading to a limitation on CO2 availability and photosynthesis would be a mechanism that is consistent with the rapid recovery of photosynthesis reported in this study and in other studies (10,32). However, we found that at all calculated intercellular CO2 levels the assimilation rates of leaves of drought stressed plants were lower than those of control plants. Similar findings have been reported by Krieg and Hutmacher (25) in sorghum. Downton et al. (10) claimed that stomatal closure in leaves of drought stressed plants is not uniform and the calculation of intercellular 47 C02 levels based on gas exchange data was inaccurate. They developed a model for calculating Ci in stressed leaves in relation to those of leaves of control plants. Assuming this model to be correct, we found that the decrease in Ci could only account for about 36.5% of the observed decrease in A of drought stressed plants while the remaining 63.5% have ix: be attributed ix: non-stomatal factors, e.g. accumulation of photoassimilates. Higher concentrations of sucrose and raffinose were detected in leaves of drought stressed cucumber' plants as compared to well irrigated plants. Although gas exchange measurements and sugar determinations were made in different experiments, these observations would be in agreement with others (2,39) who attributed the decrease in photosynthesis in drought stressed plants to the accumulation of photoassimilates in leaves. Stomatal conductance. did. not recover' completely following rewatering, probably due to the presence of ABA, which accumulates in leaves of drought stressed plants (1,12,29), at levels high enough to prevent complete stomatal opening. Fruiting cucumber plants and other crops have been reported to have higher CO2 assimilation rates than deflowered plants (5,17,28). These reports are in agreement with our results. A decrease in stomatal conductance has been suggested as the cause of the decrease in photosynthesis following fruit removal (15,31). Although we found that stomatal conductances were higher in fruiting 48 than in deflowered plants, the difference in gs is unlikely to be the cause of the observed difference in A associated with fruit bearing in cucumber plants. Based on the g5 to A relationship reported in this study, the lower stomatal conductance observed in well-watered defruited plants cannot account for the lower CO2 assimilation rates of these plants as compared to fruiting plants. On a whole plant basis, fruiting plants, despite having a smaller leaf area as compared to deflowered plants, produced a total amount of plant dry matter that was equal to the amount produced by deflowered plants. This indicates that even under conditions of drought stress, fruiting plants had a higher overall photosynthetic capacity than non-fruiting plants. Under well irrigated conditions, the higher CO2 assimilation rates of fruiting plants apparently overcompensated for the smaller leaf area resulting in a larger total amount of dry matter being produced as compared to deflowered plants. Choma et al.(5) found that, on whole plant basis, net photosynthesis and total dry matter production were similar in fruiting and deflowered strawberry plants. Fruit bearing altered the dry matter partitioning strategy of the plant. Fruits acted as strong sinks to which photoassmilates were preferentially allocated at the expense of vegetative plant parts. Similar findings for cucumbers have been reported (28). The competitive effect of fruits added to the adverse effects of water deficits in limiting the growth of vegetative 49 plant parts. The observed reductions in leaf area of drought stressed cucumber plants are in agreement with other studies (n1 field beans (24) anui sunflower(44). Drought stressed cucumber plants did not allocate more dry matter to roots as has been reported to occur in other crops (21,26). Increased sink demand in fruiting cucumber plants has been suggested to induce increases in A and in the synthesis of the translocate sugar stachyose (30). We found that stachyose levels were highest in leaves of fruiting plants, which is in contrast with the findings of Pharr et al. (30) who reported higher rates of stachyose synthesis but lower concentrations of the sugar in leaves of fruiting cucumber plants in comparison with deflowered plants. Our results indicate that when sink demand is limited, e.g. in drought stressed and deflowered plants, stachyose levels in source leaves are lower than those observed in plants in which sink demand is high. The rapid recovery of photosynthetic activity following a period of drought stress indicates that cucumber plants would be capable of recovering from mild water deficits caused by high transpiration rates under field conditions, without long term adverse effects. Mechanisms through which drought stress could have reversible effects on photosynthesis, such as reduced chloroplast volume, have already been suggested (17,22). The results of this study also indicate that the effects of water deficits and 50 fruiting on photosynthesis in cucumber plants cannot be explained solely by the observed changes in stomatal conductance. Other factors, such as accumulation of photoassimilates, apparently also impact cum CO2 assimilation in cucumber plants. 10. 11. 12. Literature cited Ackerson, R.C. 1980. Stomatal responses of cotton to water stress and abscissic acid as affected by water stress history. Plant Physiol. 65:455—459. Ackerson, R.C. and R.R. Hebert. 1981. 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The influence of age, position and environmental variables on net photosynthetic rate of sour cherry leaves. J. Amer. Soc. Hort. Sci. 107:339-344. Schulze, E.D. and A.E. Hall. 1982. Stomatal responses, water loss and C02 assimilation rates of plants in contrasting environments. Physiological Plant Ecology II. Water relations and carbon assimilation. Berlin. Springer Verlag. Encycl. Plant. Physiol. Vol 12B pp:181-230. Schulze, E.D. and M. Kuppers. 1979. Short term and long term effects of plant water deficits on stomatal responses to humidity in Corvlus avellana L. Plants 146:319-326. Sharkey, T.D. 1984. Transpiration induced changes in the photosynthetic capacity of leaves. Planta 160:143-150. Thorne, J.H. and H.R. Koller. 1974. Influence of assimilate demand on photosynthesis, diffusive resistance, translocation and carbohydrate levels of soybean leaves. Plant Physiol 54:210-207. Warrit, B., J.J. Landsberg and M.R. Thorpe. 1980. Responses of apple leaf stomata to environmental factors. Plant cell env. 3:13-22. West, D.W. and D.F. Gaff. 1976. The effect of leaf temperature and light intensity on leaf diffusion resistance and the transpiration of leaves of Malus svlvestris . Physiol Plant. 38:98-104. Yegappan, T.M. D.M. Paton, C.T. Gates and W.J. Muller. 1982. Water stress in sunflower (Helianthus Annuus L.) 2. Effects on leaf cells and leaf area. Ann. Bot. 49:63-68. CHAPTER I I Evidence for osmotic adjustment in leaf tissue of pickling cucumbers in response to drought stress Abstract Nine pickling cucumber lines including Cucumis sativus L. var. hardwickii were cultured in the greenhouse and subjected, to ‘water’ deficit treatments beginning at the onset of anthesis. Leaf water potentials of stressed plants ranged from -0.71 to -0.77 Mpa. The osmotic potentials of expressed sap of rehydrated leaves were 0.06 'U: 0.1 Mpa lower in stressed than in non-stressed plants due to solute accumulation within the tissue. No differences in the magnitude of osmoregulation were found among the cucumber genotypes; tested. Leaves of cucumber plants did not osmoregulate in response to the first drought exposure. The concentration of potassium in leaf lamina tissue (on a fresh wt. basis) of water stressed plants (82 umol/g) was 2.5 times that of control plants (33.3 umol/g). The increase 131 leaf potassium could account fer and. of the observed decrease in leaf sap osmotic potential in water stressed plants. Sucrose concentration was higher while the concentration of stachyose was lower in leaves of drought stressed plants. However, the contribution of sugars to changes 131 leaf osmotic potential was insignificant. The magnitude of osmotic adjustment in leaves of stressed plants decreased significantly within 48 hrs of rewatering 55 56 the plants. Changes in concentration of K+ and sugars in leaf lamina tissue did not account for the observed decline in solute concentration following rewatering. Introduction Osmotic adjustment increases plant tolerance to drought stress by enabling the plant to maintain cell turgor and tissue hydration.zfl: lower water potentials (2,3,8,16). A number of plant species have been shown to undergo osmoregulation in response to water deficits (1,6,20) but it has not been demonstrated to occur in cucurbits. In wheat, osmotic adjustment is a heritable trait (13) and is believed to be responsible for differences in the drought tolerance of 'wheat cultivars (5,11,12). Since cucumbers originated in the semi-arid regions of Africa and southwest Asia (6), drought tolerance or avoidance genes would be expected to be found within a diverse population of Cucumis sativus. Solutes which accumulate and contribute to osmotic adjustment include potassium, chloride and amino acids (9), betaine (10), reducing sugars (2) and non-reducing sugars (1). Organic solutes are metabolized or assimilated into other compounds following relief of water stress resulting in loss of adjustment. Consequently, the lowered osmotic potential is maintained only for a limited period of time, 57 six to ten days, after stress is relieved (15,20). The objectives of this study were to : (I) to evaluate the osmotic adjustment capacity of several pickling cucumber genotypes, (2) to identify, solutes involved in osmoregulation in cucumbers and (3) to study the maintenance of osmotic adjustment following relief of drought stress. Materials and Methods Plant material: Pickling cucumber (Cucumis sativus L.) plants were cultured during the months of May to August of 1986 and 1988 in the Plant Science Greenhouses at Michigan State 'University. Genetic lines examined in this study included Gy 14, Monoecious and gynoecious Clinton, M21, Littleleaf, Sumpter and hardwickii, a botanical variety of 9;, sativus. Seeds ‘were sown :hl 7 or 11 liter plastic containers filled with a 1:1 peat (Baccto professional mix) to sandy loam soil media depending upon the experiment. Plants were irrigated daily with a drip system and fertilized twice weekly with a 20 - 8.8 - 16.6 (N-P-K) Peter’s soluble fertilizer at 21 concentration of CLZ g/liter. Day/ night temperatures were 30 / 20C +/- 5C with no supplemental lighting provided. Plants were trained to vertical bamboo stakes and pollination was achieved using bees which were introduced into the greenhouse at anthesis. 58 Water deficit: Drought stress treatments were initiated at the onset of anthesis by withholding water from the plants for 3 to 4 days until plant water potential decreased to -0.5 to -0.8 Mpa after which stressed plants were rewatered. Stressed plants were subjected to a total of four successive drying cycles. Control plants were watered once or twice daily throughout the experiment. Fruit set on certain treatment plants was prevented by detaching the pistillate flowers from those plants daily throughout the duration of the experiment. Water potential determinations: A SoilMoisture Equipment Corp. Model 3000 series pressure chamber was used to measure the water potential of the first fully expanded leaf which usually corresponded to the fourth or fifth leaf from the shoot apex. Measurements were made between 6 and 7 A.M. The inside of the pressure chamber was lined with moistened paper towel to increase relative humidity inside the chamber and thus minimize water loss from the leaf. Following measurement of water potential, the entire leaf was immediately removed from the chamber, folded, sealed in a. plastic ‘vial and. placed. in ice for transfer to the laboratory. Osmotic potential determinations: Sections of the leaf used in ‘water' potential measurement. were rehydrated by floating on distilled water for 4 hours at 4C, then blotted dry, placed in plastic vials and stored at -20 C. After thawing the leaf tissue, the leaf was placed in the barrel 59 of a 3 cc syringe and pressed to express the leaf sap. The osmolality of the expressed sap was measured using a Wescor 5000 vapor pressure osmometer. The pressure potential of the leaf was calculated as the mathematical difference between the estimated water and osmotic potentials of that leaf. When changes in leaf osmotic potential over time were studied, leaf samples were collected at the end of the second drought stress period and at 24 and 48 hours after the plants were rewatered. Heat girdling: Leaf petioles were heat girdled to block phloem transport by passing hot air, at a temperature of 65C, over a 4 cm region of the petiole for 3-5 minutes. Leaf sections were collected prior to and 24 hours after girdlimg. All plants were rewatered immediately following girdling. Osmotic potentials of the leaf sections were then determined. Leaf sugar and. potassium (determinations: Leaf samples were freeze dried for 24 hours then finely ground with a mortar and pastel. Sugars were extracted from tissue subsamples (0.2 g) with 80% ethanol at 70 C for 1 hour. The extract was filtered through a 1km :l Whatman filter paper and the ethanol evaporated. The residue was redissolved in 25 ml of deionized water and an aliquot of the resultant solution filtered through a 0.45 um Millex-HA filter unit. Sugars and sugar alcohols were separated and were assayed using a Dionex Carbopac PA1 anion exchange separation column with a Dionex series 40001 High Performance Ion 6O Chromatography Module and a pulsed amperometric detector with a Gold electrode. A 0.1 M NaOH solution was used as the eluant. Potassium concentration in leaf tissue extracts were determined by standard procedures using atomic emission spectrophotometry (Instrumentation Laboratory, Video 12). Results Osmotic potentials in leaves from drought stressed plants ranged from -0.71 to -0.77 MPa as compared to -0.64 to - 0.68 MPa in leaves of well watered plants (Table 1). These osmotic potential differences are believed to reflect differences in solute accumulation in leaf lamina tissue since the leaves had been rehydrated prior to measurement of leaf osmolality. Osmotic potentials did not vary among the inbred lines tested. Osmotic adjustment, calculated as the mathematical difference between tflmz leaf osmotic potentials of stressed and non-stressed plants, was similar in all genotypes tested. Plant water potentials at the end of the three water deficit cycles ranged between -0.48 and -0.73 Mpa (Data not shown), which represented a moderate level of stress in the cucumber plants. Differences between leaf osmotic potentials of drought stressed and control plants ranged between 0.03 and 0.09 Mpa and were significant only after the second and third exposures to water deficit (Table 2). 61 Table 1. Leaf osmotic potentials of eight pickling cucumber genotypes exposed to drought stress or irrigated during fruit development. z x Osmotic potpptial (Mpal Osmotic Drought potential Genotype stressed Irrigated difference G. Dwarf 2780 -0.70 -0.64 0.06 Gy14 -0.75 -0.66 0.09 G. Clinton -O.75 -0.68 0.07 M. Little Leaf —0.77 -0.67 0.10 M. Clinton -0.76 -0.68 0.08 M 21 -0.71 -0.65 0.06 C.sativus var hardwickii -0.74 -0.66 0.08 Sumpter -0.73 -0.65 0.08 Mean -0.74 -0.66 Significance Cultivar NS NS Stress *** __ Cultivar X Stress NS 2. Leaf samples were collected at the end of the second drought exposure. y. G. and M. indicate a gynoecious or monoecious flowering, respectively. x. Mathematical difference between the leaf osmotic potentials of stressed and well irrigated plants. ***, NS. Siginficant at the 0.1% probability level and not significant, respectively. 62 Table 2. Osmotic potentials of cucumber leaves during three exposures to drought stress under greenhouse conditions. Leaf osmotic pptential IMPa)Z Stress exposurey Water regimen First Second Third Drought stressed 0.73 0.65 0.72 Well-watered 0.70 0.56 0.66 F-significance NS * * z. Osmotic potential of the sap of the 4th or 5th leaf from the shoot apex of cucumber plants. Leaves were re- hydrated prior to osmotic potential determination. y. Plants were not watered until leaf water potential had reached about -0.6 Mpa; plants were then rewatered and another stress exposure was initiated. NS.Not significant at the 5% probability level. 63 Stressed leaves had an expressed sap osmolality of 306.7 mmolal whiCh was hdgher than that of non-stressed leaves (Table 3). Reducing sugars, representing the sum of the concentrations of glucose, fructose and galactose, were present at similar levels in leaves of stressed and non-stressed plants. Under both water regimens, reducing sugars accounted for about 2% of the total leaf sap osmolality. Sucrose was present at a concentration of 2.2 umol/g fresh wt. in leaves of drought stressed plants which represented a contribution of 0.73% to the total leaf osmolality. Sucrose concentration in leaves of control plants was less than half the concentration detected in leaves of stressed plants. In well-watered plants, stachyose was detected at a concentration of 2.5 umol/g fresh wt., 0.8 umols higher than the level detected in leaves of stressed plants. Potassium was found at a higher concentration and made a contribution of 26.8% to leaf osmotic potential in leaves of stressed plants, as compared to 12.3% in leaves of control plants. Continuously well- watered plants maintained solute potentials of -0.63 to -0.65 Mpa throughout the 48 hour period while the solute potentials of stressed plants increased from -0.72 to -0.66 Mpa (Table 4). Forty- eight hours after' stressed. plants ‘were rewatered, significant osmotic potential differences were still evident in the leaves of these plants. Within 48 hrs of rewatering, leaf osmotic potential in fruiting plants increased from 64 -0.7 to -0.64 Mpa” 'while :hi deflowered plants it increased from -0.67 to -0.65 Mpa during the same period of time. The concentration of reducing sugars, sucrose and stachyose decreased within the 48 hours following rewatering while no change was observed in the potassium level (Table 5). The cumulative changes observed in solute concentration within 48 hours following rewatering were insignificant in comparison with the observed changes in leaf sap osmolality within the same period. It was observed that changes in assayed solute concentrations following rewatering were similar in fruiting and defruited plants (Data not shown). Within 24 hours of girdling the leaf petiole, stressed leaf osmotic potential decreased by 43.3 mmolal while the potassium concentration in the same leaves increased from 98.3 to 154.7 umol/g fresh wt., a change of 56.4 umol/g (Table 6) which could account for 100% of the increase in leaf sap osmolality following petiole girdling. Other inorganic solutes imported via the xylem would be expected to have proportional increases in concentration. In non- stressed leaves, the increase in potassium concentration accounted for 52.8% of the increase in leaf sap osmolality following girdling. Table 3. 65 Leaf osmolality and concentration of selected solutes in cucumber leaf lamina tissue under drought stress and well irrigated conditions. Solute concentration (umol/g fresh wt.) Z Leaf sap Reducing Treatment osmolality sugars Sucrose stachyose K+ (mmolal) Stressed 306.7 5.7 2.2 1 7 82.0 Non-stressed 271.3 5.5 1.0 2.5 33.3 F-Significance *** NS * * ** 2. Mathematical sum of concentrations of glucose, fructose and galactose. NS.Not significant at the 5% probability level. 66 Table 4. Changes in cmcumber leaf osmotic potentials following relief from water stress. Leaf osmotic potential (Mpa) y 2 Time after Water regimen_ Fruit bearing rewatering Stressed Irrigated Fruiting Defruited w 0 hrs -0.72 -O.65 -0.70 -0.67 24 hrs -0.68 -0.63 -0.65 -0.66 48 hrs ~0.66 -0.63 -0.64 -0.65 F- significance Fruiting X Time after rewatering __ ** Water regimen X Time after rewatering ** z. Stressed plants were subjected to 2 drought stress cycles and measurements were made at the end of the second stress cycle. y. Plants were either defruited by removal of pistillate flowers or allowed to set fruits. x. The mathematical difference between the leaf osmotic potentials of drought stressed and well watered plants. w; Leaf lamina ‘tissue. samples ‘were collected. prior' to rewatering of plants. **.Significant at the 1% probability level. Table 5. 67 Changes in solute concentrations in leaf lamina tissue of drought stressed cucumbers following rewatering. Solute concentration (mg/g fresh wt.) Time after Reducing rewatering sugars Sucrose Raffinose Stachyose K+ z 0 hrs 5.5 0.35 1.71 65.4 24 hrs 5.5 0.25 1.73 69.2 48 hrs 3.8 0.22 1.34 72.3 L.S.D (0.05) 1.3 NS 0.20 NS 2. Leaf tissue was sampled just before rewatering of plants and rehydrated. by floating leaf sections distilled water for 4 hours at 4C. NS. Not significant at the 5% probability level. on , _,_£ 68 Table 6. Changes in solute relations in heat girdled leaves of drought stressed and well irrigated cucumber plants following rewatering. Time Leaf sap Potassium Treatment (hours) osmolality (umol/g (mmolal) fresh wt) Drought stressed 0 307 98.3 Well irrigated 0 280 46.2 Drought stressed 24 351 154.7 Well irrigated 24 347 81.2 L.S.D (0.05) interaction 20 16.4 F-significance Water regimen * *** Time after girdling *** *** Water regimen X Time after girdling 0.09% 0.07% 2. Leaf tissue was sampled prior to and 24 hrs after girdling. *, and ***. Significant at the 5% and 0.1% probability levels, respectively. Discussion Pickling cucumbers undergo osmotic adjustment in response to drought stress based on the observed osmotic potential differences between drought stressed and well watered plants. The magnitude of the adjustment ranged between 0.06 and 0.1 MPa and was reproducible in replicated repeated experiments. In comparison, osmotic adjustments of 0.1 to 0.4 Mpa have been reported for agronomic crops, e.g. maize and sorghum (1,19). Genotypic differences in osmoregula- tory capacity have been reported for other crops (7, 11, 12, 21). However, in Cucumis sativus L., the capacity for osmotic adjustment does not appear tx> vary among the various genotypes tested. The genotypes used in this study were of a relatively narrow genetic base of inbred lines that had been bred for growth under optimal cultural conditions. Because of their limited genetic diversity, these genotypes did not exhibit the variability and the magnitude of the response needed for osmotic adjustment to have a significant impact on the degree of drought tolerance in pickling cucumbers. A more diverse pool of cucumber genotypes should be investigated in order to identify’ cucumbers 'with 21 higher degree: of drought tolerance. Cucumber plants did not exhibit changes in osmotic potential in response to the first exposure to drought stress indicating' that. prior exposure to water deficit might be needed before osmoregulation could occur. The magnitude of osmotic adjustment capacity has been 69 70 reported to increase with repeated exposure to water deficits (14,15). Our results indicate that cucumber leaves have a limited capacity for osmoregulation which did not allow for an increase in the magnitude of osmotic adjustment following the second drought stress exposure. The increase in the concentration of potassium in leaves of stressed plants could account for 100% of the decrease in leaf osmotic potential as a result of drought stress, while the contribution of sugars was insignificant. Handa et al. (1983) found that in tomato cell cultures, potassium contributed 13.8% of the total cell osmotic potential while sugars contributed about 20% of the osmotic potential, much higher than levels observed in our study. Potassium, chloride and amino acids have been reported to account for 80% of the decrease in osmotic potential in stressed leaves, while sugars accounted for the remaining 20% (9). In contrast, reducing sugars (2) and non-reducing sugars (1) were reported as the main solutes that accumulated in stressed leaves of maize and sorghum. However, the levels of sugars we detected in leaf tissue are comparable to those reported for cucumbers (17) which suggests that sugars do not have a major role in osmotic adjustment in cucumbers. The magnitude of the difference between the osmotic potentials of stressed and non-stressed leaves was found to decrease with time after rewatering, which is in agreement with others (14,15). However, the rate of change 71 in osmotic potential difference was higher in the current study. Changes in the concentrations of assayed solutes could not account for the observed increase in leaf osmotic potential following rewatering. The increase in leaf sap osmolality following heat girdling of the petiole could be accounted for by the increase in the concentration of inorganic ions. Fruiting influenced the maintenance of osmotic adjustment in leaves. The more rapid decline in solute concentration in leaves of fruiting plants following rewatering suggests an effect of fruits on leaf solute redistribution. An effect of fruits on osmoregulation was implied by Ackerson (1981) who suggested that solutes accumulated in leaves of stressed plants as a result of a decreased sink capacity. Resumption, of fruit «growth at 61 higher rate after rewatering would increase sink strength and the demand for solutes out of leaves. Fruits had no apparent effect on the levels of assayed solutes in stressed leaves following rewatering. The observed effect of fruiting on leaf osmotic potential following rewatering is probably due to effects on other solutes not assayed in this study. Osmotic adjustments of 0.06 to 0.08 Mpa were apparent in leaves of drought stressed cucumber plants and K+ appears to be the major ion contributing to osmoregulation. No re- + in leaf tissue of drought distribution of accumulated K stressed plants was observed following rewatering. Consequently, the increase in leaf osmotic potential of 72 drought stressed plants following rewatering must have been due to changes in the concentrations of other solutes. 6. 7. 10. 11. Literature Cited Acevedo, E., E. Fereres, T.C. Hsiao and D.W.Henderson. 1979. Diurnal growth trends, water potential and osmotic adjustment of maize and sorghum leaves in the field. Plant Physiol. 64: 476-480. Ackerson, R.C. 1981. Osmoregulation in cotton in response to water stress. II. Leaf carbohydrate status in relation to osmotic adjustment. Plant Physiol 67:489-493. Ackerson, R.C. and R.R. Hebert. 1981. Osmoregulation in cotton in response to water stress. I. Alterations in photosynthesis, leaf conductance, translocation and ultrastructure. Plant Physiol. 67:484-488. Blum, A., J. Mayer and G. Gozlan. 1983. Associations between plant production and some physiological components of drought resistance in wheat. Plant Cell Env. 6:219-225. Boyer, J.S. 1971. Non-stomatal inhibition of photosynthesis in sunflower at low leaf water potentials and high light intensities. Plant Physiol. 48:532-536. Esquinas-Alcazar, J.T. and P.J. Gulick. 1983. Genetic resources of cucurbitacea. International Board for Plant Genetic Resources. Rome. 101 pp. Fanjul, L. and P.H. Rosher. 1984. Effects of water stress on internal water relations of apple leaves. Physiol Plant. 62:321-328. Fisher, R.A. and M. Sanchez. 1979. Drought resistance in spring wheat cultivars. II Effects on plant water relations. Aust. J. Agric. Res. 30:801-814. Flower, D.J. and M.M. Ludlow. 1986. Contribution of osmotic adjustment to dehydration tolerance of water-stressed pigeonpea (Cajanus caian (L.) Millsp.) leaves. Plant Cell Env. 9:33-40. Handa, S.R., R.A. Bressan, A.K. Handa, N.C. Carpita, P.M. Hasegawa. 1983. Solutes contributing to osmotic adjustment in cultured plant cells adapted to water stress. Plant Physiol. 73:834-843. Hanson, A.D. and W.D. Hitz. 1982. Metabolic responses of mesophytes to plant water deficits. Ann. Rev. Plant. Physiol. 33:163-203. 12. Keim, D.L. and W.E. Kronstad. 1981. Drought response 73 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 74 of winter wheat cultivars grown under field stress conditions. Crop Sci. 21:11-15. Morgan, J.M. 1977. Differences in osmoregulation between wheat genotypes. Nature 270:234-235. Morgan, J.M. 1984. Osmoregulation and water stress in higher plants. Ann. Rev. Plant. Physiol. 35:299-319. O’Neill, S.D. 1983. Role of osmotic potential gradients during water stress and leaf senescence in Fragaria Virginiana. Plant Physiol. 72:931-937. Oosterhuis, D.M. and S.D. Wullschleger. 1987. Osmotic adjustment in cotton (Gossypium hirsutum L.) leaves and roots in response to water streess 84:1154-1157. Parsons, L.R. and T.R. Howe. 1984. Effects of water stress on the water relations of Phaseolus vulgaris and the drought resistant Phaseolus acutifolius Physiol. Plant. 60:197-202. Pharr, D.M., S.C. Huber, and H.N. Sox. 1985. Leaf carbohydrate status and enzymes of translocate synthesis in fruiting and vegetative plants of Cucumis sativus L. Plant Physiol. 77:104-108. Raschke, K. and R. Hedrich. 1985. Simultaneous and independent effects of abscisic acid on stomata and the photosynthetic apparatus in whole leaves. Planta 163:105-118. Sanchez-Diaz, M.F. and P.J. Kramer. 1978. Turgor differences and water stress in maize and sorghum leaves during drought and recovery. J. Exp. Bot. 24:511-515. Shahan, K.W., J.M. Cutler and P.L. Steponkus. 1979. Persistance of osmotic adjustment in rice. Hortscience. 14:404 (Abstr.). Stout, D.G. and G.M. Simpson. 1978. Drought resistance of Sorghum bicolor. I. Drought avoidance mechanisms related to leaf water status. Can. J. Plant. Sci. 58:213-224. CHAPTER II I Water deficit effects on pickling cucumber plant growth, fruit productivity and quality Abstract 1&1 greenhouse experiments, eleven :monoecious and gynoecious pickling cucumber parental lines and F1 hybrids with different vine types were subjected to water deficits during the flowering and fruiting growth stages. In all genotypes tested, drought stress reduced plant productivity. Water stressed plants set 32 to 42.3% fewer fruits and had 25.5 to 46.4% lower total fruit dry weight than non-stressed plants during a three week harvest period. Fruits from stressed plants were significantly shorter and had lower LD ratios than fruits from non- stressed plants. The incidence of incomplete seed set increased in fruits of drought stressed plants. Water deficits had 1K) apparent effect (M) the incidence of misshapen fruits. Fruit growth rate was reduced by water deficits; the first fruit set CH1 a plant needed about 2 days longer to reach a diameter of 42 mm. It was estimated that 33% of the decrease in fruit growth rate was due to water supply limitation, while assimilate supply limitation accounted for 67% of the total decrease in fruit growth. Water deficits did not alter the fruiting bearing pattern of cucumber plants. The distribution of fruit harvest over the three week harvest period was similar in 75 '111111. .l 76 stressed and non—stressed plants. Vegetative biomass (on a dry wt. basis) of water stressed plants was 20.8 to 38.8% lower than those of non-stressed plants. It is concluded that, under the experimental conditions of this study, the genotypes tested have a low drought tolerance. Introduction Water deficits adversely affect cucumber plant growth (14) which can ultimately result in fruit yield reductions in cucumbers (3). The flowering and fruiting period has been identified as an important stress- sensitive growth stage in plant development as related to crop productivity. Decreases in fruit productivity under conditions of water deficit have been attributed to ovule abortion, and consequently low fruit set, poor seed set and slow expansive growth of fruits (7,17,18). Machine harvested pickling cucumbers are mainly grown under rainfed conditions. In the midwestern United States, cucumber crops are exposed frequently to temporary droughts of 7 to 10 days during the summer months (16) which reduce potential fruit yield and quality (5,8). Limited research has been conducted on the responses of pickling cucumbers to drought stress. This study was conducted with the following objectives; to study the effects of water deficits on cucumber plant growth, fruit productivity and quality, and to identify genotypic 77 differences in responses to drought that might exist among several cucumber parental lines and cultivars. Materials and Methods Greenhouse experiments were conducted at Michigan State University during mid summer of 1986 and 1987 and during April and May of 1988. Plant.:materialzPickling' cucumber' (Cucumis sativus L.) seeds 'were sown in 21 1:1 peatmoss to sandy loam soil mixture in 11-liter containers. Plants were fertilized twice weekly with a 20- 8.8- 16.6 (N-P-K) Peter’s soluble fertilizer at 0.2 g/l. At anthesis, a beehive was placed in the greenhouse to facilitate pollination. Temperatures were maintained at 30 +-5C during the day and 20 +-5C at night and no supplemental lighting was provided. During the vegetative growth stage, all plants were watered daily. papa; deficit: Drought stress treatments were initiated at the onset of anthesis by withholding water from the plants for a period of 3 to 4 days until plant water potential had decreased to —0.6 to —0.8 Mpa. Plant water potential measurements were made at dawn using a Soilmoisture Equipment Corp. pressure chamber. Control plants were watered daily throughout the experiment. Fruit length and diameter were measured daily following fruit set. Fruits were harvested when they reached a diameter of 50 +/- 3mm, weighed and internal fruit 78 characteristics evaluated and measured. Determinations of fruit dry matter content were also made by dehydrating fruit samples in an oven at 65C for 72 hrs. Genotype evaluation: Seven pickling cucumber genotypes, gynoecious dwarf 2780, Gy14, gynoecious Clinton, M21, M21x Gy14 F1, M21 x Clinton F1 and monoecious Clinton (Campbell Institute for Agricultural Research, Napoleon, Ohio) were included in this experiment. These genotypes are either parental lines of" numerous commercial cultivars or F1 hybrids. Plants were subjected to several drought stress cycles during the flowering and fruiting growth stages. Fruits were harvested when they reached 5 +/- 0.3 cm for a three week period. Individual fruit length, diameter and fresh and: dry weights as well as total number of fruits harvested per plant were recorded throughout the period. Above ground vegetative plant parts were harvested and dry weights determined after final fruit harvest. Fruit volume was estimated from length and diameter measurements assuming a cylindrical fruit shape. A fruit density factor was calculated by dividing_the final fruit dry weight by final fruit volume. Percent dry matter and fruit density have been shown to remain constant during cucumber fruit ontogeny (15) except when placental hollows or carpel separation occur within a fruit. Fruit dry weight used in plotting fruit growth curves were obtained by multiplying density factor by the daily fruit volume. No significant difference in density was found between stressed and non- 79 stressed fruits (Data not shown). A mean daily fruit growth rate (fresh weight basis) was calculated by dividing the mathematical difference between the final fruit fresh weight and the estimated initial fresh weight by the number of days to harvest. A similar rate was calculated for the increase in fruit dry weight. _R_e_fl1_l_t_s_ Plants grown during the summer of 1986 were characterized by extensive leaf and shoot growth which was probably due to an above average number of cloudy days during the growing period. During the summer of 1987, there were fewer cloudy days and vegetative plant growth was less extensive as compared to 1986. Drought stress during flower and fruit development adversely affected both vegetative and reproductive growth in pickling cucumber plants during both years of experimentation. The amount of total leaf and stem (shoot) biomass produced by drought stressed plants was 34% to 53% lower than that produced by well-watered plants of the same genotype (Table 1). Reductions in fruit biomass production due to drought stress ranged between 19 and 54% in 1986 (Table 1) and approximately 39% in 1987. The lower fruit biomass production under drought conditions could largely' be attributed to fewer fruits being set on the plants and the slower expansive fruit 80 growth rates. In 1987, only approximately 3.1 fruits set on each plant during a three week period under drought conditions as compared to 4.9 fruits per plant in irrigated plants (Table 2). Drought induced plant water deficits also delayed fruit maturation, fruits reaching harvestable size (5 cm diameter, 275 ml volume), by 2 days (Figure 1). Expansive fruit growth rates under drought stress conditions, expressed on a volume basis, were significantly lower than the growth rates of fruits from well irrigated plants as evidenced by the lower slopes of the fruit volume X time regression curves (Figure 1). All the pickling cucumber genotypes evaluated responded similarly to drought stress. Although there was a statistically significant interaction between genotype and water regimen in 1986, the interaction was primarily due to large vegetative growth and fruit productivity differences among the genotypes under well irrigated conditions (Table 1). Gynoecious Clinton and hardwickii, however, produced the highest and lowest fruit biomass (dry weight basis) respectively, under both water regimens. 81 Table 1. Effects of drought stress on dry matter pro- ductivity in selected pickling cucumber genotypes (1986). Dry weight(g.plant’1) Z Y Shoot dry weight Fruit dry weight Genotype Well- Drought- Well Drought watered stressed watered stressed G. Dwarf 46.0 24.8 52.7 29.3 Gy14 35.0 22.9 45.2 20.7 G. Clinton 38.0 21.7 64.6 30.1 G. Littleleaf 37.1 23.7 50.7 21.7 M. Clinton 60.0 32.1 28.3 23.0 M 21 39.9 26.2 41.1 25.4 M. Littleleaf 51.4 29.0 29.5 15.2 Sumpter 47.4 28.1 41.6 21.7 Hardwickii 72.4 44.8 17.8 8.2 L.S.D(0.05) Drought stress X Genotype *** ** 2. Weight of leaves and stems after the final fruit harvest. y. Fruits were multiple harvested for a period of 3 weeks. x G. and M. indicate gynoecious and monoecious flowering, respectively. *** and **. Significant at the 0.1% and 1% probability levels, respectively. 82 Table 2. Effects of genotype and water deficits on fruit productivity productivity and quality of greenhouse grown pickling cucumbers (1987). Z Y Fruit pgoductivitv Percent pp fruits number/ total d.wt Incomplete plant g/plant misshapen seed set Qultixar G. Dwarf 4.2 29.4 27.9 29.7 Gy14 4.7 32.8 39.3 22.1 G. Clinton 3.8 28.9 18.8 21.4 M21 4.1 31.6 7.8 28.4 G.Clintonx M21 4.4 34.0 13.6 24.8 Gyl4 X M21 4.2 30.0 21.9 22.2 M. Clinton 2.8 24.6 0.0 18.2 x L.S.D (0.05) 0.7 NS 10.9 NS Water regimen Drought Stressed 3.1 22.8 17.1 32.2 irrigated 4.9 37.6 15.9 15.3 x L.S.D (0.05) 0.4 3.3 NS 6.7 NS. Not significant at the 5% probability level z. Fruits were harvested for aperiod of 3 weeks. y. Seed set was evaluated visually for presence of aborted ovules. x. No genotype X water regimen interaction was found and L.S.D values are for comparison of genotypes and water regimen. 83 630% etc _o>o_ N0 9: 00 SEE oocoptcoooaocm :3: Horacio 0 055.530 pco 50:2 pco coooEBp #3.: to mucoEocsmooE xzop Eot poLoSoBo mo; oE:_o> tam .30.. 530cm #3.: co 53:03 .65; Lo 3035 .F .911 m>00 s ... 1. a a. s . ommobmlcoz o x \\\.x 0 commobm x x H\ \\1... WM wm\.\ \\ \ \ x x x m\“ \V \ j .\ \\\ X \ \\\ .1. x a. \ m . X x \ J \\\m \ n x \\ \\ m 1009 I. \ \\\ 1. x \ \\ m \\\ x A X \\ \ m 0 X \“\ \\ m a mi. x \\ m \\ w x WM \ \ m R m 8 \W\\\ x \\\m D x \\\\\\ x \\\D o ICON \W/ \\ \ \ \\\ m n x A \ \ X\.\\\\ X W“W\ m D \ \ \\ X X\“\W\ D D r. \\\ \ X D 6 SA . 53% . as + e: \ Em 1 » 000 84 In the 1987 experiment, the genotypes did not differ in total fruit dry weight per plant (Table 2), and were equally affected by the induced drought stress. In terms of number and timing of fruit set, genotypic differences were apparent. Monoecious Iittleleaf and hardwickii in 1986 (Data not presented) and monoecious Clinton in 1986 (Table 2) set the fewest fruits per plant. Typically, the gynoecious genotypes were the higher yielders when evaluated on a fruit biomass basis especially under irrigated. conditions. Gynoecious plants ‘were also observed to set fruit earlier than monoecious plants. Although both flowering‘ habits/ genotypes exhibited cyclical fruit setting patterns, the percentage of total fruit harvested during each of the first two maturation cycles was nearly equal for the gynoeciuos genotypes (e.g. G. Dwarf: Figure 2) under both water regimens. Monoecious lines, in contrast, set only approximately 10% of their fruit production for the three week period during the first cycle. The incidence of misshapen fruits produced by cucumber plants did not increase in response to water deficits, but were influenced by genotype. No misshapen fruits were produced by monoecious Clinton plants while 39.3% of fruits harvested from Gy14 plants were misshapen (Table 2). 85 .>_o>:ooamoc .mctoso: m:o_ooocoE pco 9566053 3 cocoa .2 ace .0 ...BoEEp E EE0+00 core, cosmoio: 8oz, BEE .poroa “mead: some/IRES o 86 W36: :3: .6 55.5ng .N .9... coop EmoZoI Om. 0N ON 0_ O— 0 O n n L l-l LILI h n — L. Li 1P Lll — h h h h i—li- p p P —L1||r be me... 0 s me We . 1 M. WON mag r ”HO 1 D .1: T 110% M m1 a 8.1 1 $01 816: as: .38 86385 Nu . W COLE—O .O mnfldc :3: .20» 20.350 ESE T00 -3: memes s swims . m. WON “MHZ 1 1 U10 1 D I! 19. mm 1 m 5.01.. 0N1.o: 23.: 300% 638ch N 1 «new Co E . .... . . b ..0 2 male: La: .33 soseoo Es. Woe 86 The percentage of misshapen fruits produced by other genotypes in this study ranged between 7.8% anui 27.9%. The incidence of fruits with aborted ovules and consequently limited seed set more than doubled in response to drought stress but no differences were found among the genotypes tested. Fruits of non stressed plants were, on average, 15.8 mm longer and had larger LD ratios than stressed fruits of equivalent diameter. Because fruits were harvested when they reached a diameter of EH) +/- 3 mm, diameters of stressed and non-stressed fruits were not significantly different (Table 3). The seed cavity diameter in drought stressed fruits, expressed as a percentage of the fruit diameter, was 1.9% smaller than that of well-watered fruits. I The rate of fresh weight increase for stressed fruits was 29.9% lower than that of non-stressed fruits (Table 4) but only 19.9% lower when expressed on a dry weight basis. The second fruit set on non-stressed plants grew more slowly than the first fruit set on the plant. In stressed plants, the growth rates of the first and second fruit were similar to that of the second fruit set on watered plants, resulting in an interaction between water regimen and fruit number. 87 Table 3. Effects of water deficits on cucumber fruit dimensions. Fruit Fruit 2 Water length diameter LD Seed cavity regimen (mm) (mm) ratio (% of diameter) I Drought stressed 115.4 48.0 2.40 47.4 Well-Watered 131.6 49.5 2.66 49.5 F-Significance *** NS * * Z. Ratio of fruit length to fruit diameter. NS. Not significant at the 5% probability level. 88 Table 4. Effects of water stress and fruiting sequence on cucumber fruit growth rates. 2 __Growth rate (g/dayl__ Water Fruit Fresh weight Dry weight regimen number basis basis Watered 1 28.4 1.42 Watered 2 21.9 1.11 Stressed 1 19.9 1.14 Stressed 2 19.9 1.14 L.S.Dy(0.05) 2.7 0.14 Stress x fruit number *** ** z. Fruit no. 1 and 2 are the first and second fruits set on the plant. y. L.S.D for the stress x fruit number interaction. *** and **. Significant at the 0.1% and 1 % probability levels, respectively. Discussion Water' deficits had adverse effects on cucumber plant growth and. productivity. None of the genotypes studied exhibited drought tolerance as they suffered decreases in vegetative and reproductive growth upon exposure to drought stress. Water deficits are known to cause reductions in photosynthetic rates by as stomata close in stressed plants (6). The combined effects of a smaller leaf area and reduced photosynthetic rates would limit a stressed plant’s capacity to produce dry matter. This is reflected by the observed decrease in total dry matter produced in water stressed plants. The decrease in fruit dry matter production in drought stressed plants was mostly due to the decrease in the number of fruits set by these plants. Fruits on stressed plants frequently ceased expansive growth at some point following pollination. Pollen viability is known to decrease upon dehydration (9). Pollen used in pollination in the current study, however, was obtained from well watered plants and was viable, as indicated. by the successful pollination. on :non-stressed plants. However, a dehydrated stigma, as was probably the case in stressed flowers, might have retarded pollen germination or slowed down pollen tube elongation which ultimately and lead to a decrease in the percentage of ovules being fertilized. Seed set and development would have been poor under such conditions, which is consistent 89 —‘— 90 with that observed in fruits of drought stressed plants. The genotypes tested did not differ in their capacity to support seed set and development under conditions of drought stress. Although fruit set and the total number of fruits set was decreased by water deficit, drought stress had no apparent effect on the fruit bearing pattern of cucumber plants. Drought stressed cucumber plants were apparently unable to support fruit growth until a certain number of days had eiapsedaafter the first fruit was harvested and consequently no shift toward earlier fruit set was observed. A high LD ratio is a desirable characteristic in pickling cucumber fruits. Drought stress reduced fruit LD ratios by limiting fruit elongation. The reason for such an effect has not been identified. However, gradients in water potential within the fruit might be expected, with lower water potentials at the blossom end of the fruit which is furthest away from the water source, the peduncle. Fruits on drought stressed. plants were frequently observed to develop tapered ends at the blossom end, and in severely stressed plants, shrinkage of fruit tissue also started at the blossom end of the fruit. The observed decrease in seed cavity diameter might be due to poor seed set in stressed fruits which probably led to a decrease in the production of growth promoting hormones needed for placental tissue growth. limited fruit elongation under conditions of drought stress might also be associated with a reduced 91 supply of hormones from developing seeds. The contributions of water limitation and assimilate supply limitation on fruit growth were estimated using data on fruit growth rates. Expressed on a dry weight basis, stressed fruits grew 19.9% slower than non stressed fruits; this probably represents the direct effect of assimilate supply limitation. On a fresh ‘weight basis, growth rate of stressed fruits was 29.9% lower than that of non stressed fruits. The difference between the dry weight and fresh weight basis percentages might reflect the direct contribution of water limitation.cnl cell expansion and the increase in fruit size. Following the harvest of the first fruit set on a plant, assimilates available for fruit growth apparently become limiting by the time the second fruit is set on a cucumber plant. The first fruit set on the plant probably depleted stored assimilates and limited leaf growth (1,15). The growth rate of the second fruit on well watered plants was similar to that of fruits of stressed plants where photosynthesis is limited by water deficit. Increased competition between fruits and other sinks in the plant probably contributed to the decline in dry weight gain in the second fruit. It was observed that in stressed plants, small misshapen developing fruits regained normal shape as they increased in size, probably due to rewatering of stressed plants following each stress exposure. Similar observations were 92 made by Kanahama and Saito (12) in cucumber plants which had been partially defoliated. This might explain the lack of an effect of drought stress on the incidence of misshapen fruits. Differences in plant morphology and in sex expression had no apparent effect on the degree of drought tolerance in pickling cucumbers. Vegetative shoot growth was extensive under greenhouse conditions and vines were generally larger than typical field grown plants of similar genotypes. However, vine growth characteristics, e.g. gynoecious dwarf 2780 vs Gy 14 which had extensive vine growth, had no apparent effect on the response of cucumber plants to the water deficits. Both, monoecious and gynoecious genotypes were equally susceptible to drought stress. The delayed fruiting habit in monoecious genotypes allowed them to produce a larger leaf area but did not improve their tolerance to drought stress, expressed on the basis of fruit and total biomass productivity. The genotypes tested in this study had a relatively similar genetic background which might explain the lack of a difference in their response to drought stress. _C__._ sativus var. hardwickii, which was the only non-commercial genotype included in this study did not exhibit drought tolerance. However, characteristics which might confer drought tolerance in hardwickii and other genotypes under field conditions, such as rooting pattern, might have been suppressed under greenhouse growing conditions. The results 93 of this study also indicated that inbred parental lines used in development of commercial cultivars have been bred for growth under optimal environmental conditions. Consequently, the potential for improvement of drought tolerance in currently available commercial cucumber genotypes is apparently limited. Increasing drought tolerance of pickling cucumbers would probably require the utilization of pp sativus germplasm from arid and semi- arid regions of the world. 1. 10. 11. 12. Literature Cited Barrett, J.E. III and H.J. Amling.1978. Effects of dpveloping fruits on production and translocation of C 4labelled assimilates in cucumber. Hortscience 13:545-547. Choma, M.E., J.L. Garner, R.P. Marini, and J.A. Barden. 1982. Effects of fruiting on net photosynthesis and dark respiration of ’Heaker’ strawberries. Hortscience 17:212-213. Cummins, T.L. and D.W. Kretchman. 1975. Relation of internal water to growth and development of pickling cucumber. Hortscience 10:319 (Abstract). DeJong, .1986. Fruit effects on photosynthesis in Prunus persica. Physiol. Plant. 66:149-153. Doss, B.D.m C.E. Evans, and J.L. Turner. 1977. Irrigation and applied nitrogen effects on snap beans and pickling cucumbers. J. Amer. Soc. Hort. Sci. 102:654-657. ' Downton, W.J.S., B.R. Loveys, and W.J.R. Grant. 1988. Stomatal closure fully accounts for the inhibition of photosynthesis by abscisic acid. New Phytologist 108:263-266. Dubetz, S. and P.S. Mahalle. 1969. Effect of soil water stress on bush beans Phaseolus vulgaris L. at three stages of growth. J. Amer. Soc. Hort. Sci. 94:479-481. Elkner, K.,1986. The relationship between the occurence of cavities in cucumber fruits and some of their morphological characteristics. Biuletyn Warzywniczy 26:383-395. Gay, G., C. Kerhoas, C. Dumas. 1987. Quality of a stress-sensitive Cucurbita pepo L. pollen. Planta 171:82-87. Hall, A.J. 1977. Assimilate source-sink relationships in Capsicum annuum L. I. Thedynamics of growth in fruiting and deflorated plants. Aust. J. Plant Physiol.4:623-636. Huck, M.G., K.I. Ishihara, C.M. Peterson, and T. Ushijima. 1983. Soybean adaptation to water stress at selected stages of growth. Plant Physiol. 73:422-427. Kanahama, K. and T. Saito. 1985. Curvature occurence and its recovery in cucumber ovary and fruit. J. Jap. Soc. Hort. Sci. 54:357-363. 94 l3. 14. 15. l6. 17. 18. 19. 95 Krizek, D.T., A. carmi, R.M. Mirecki, F.W. Snyder, and J.A. Bunce. 1985. Comparative effects of soil moisture stress and restricted root zone volume on morphogenetic and physiological responses of soybean (Glycine mag). J. Expt. Bot 36:25-38. Ortega, D.G. and D.W. Kretchman. 1982. Water stress effects on pickling cucumber. J. Amer. Soc. Hort. Sci. 107:409-412. Pharr, D.M., S.C. Huber, and H.N. Sox. 1985. Leaf carbohydrate status and enzymes of translocate synthesis in fruiting and vegetative plants of Cucumis sativus L. Plant. Physiol. 77:104-108. Ruffner, J.A. and F.E. Bair. 1977. The weather almanac. Gale Research Company. Book Tower, Detriot. ppz49S-497. Schaffer, B., J.A. Barden, and J.m. Williams. 1986. Partitioning of [ 14C] photosynthate in fruiting and debolossomed day- neutral strawberry plants. Hortscience 20:911-913. Shen, X.Y. and B.D. Webster. 1986. Effects of water stress on pollen of Phaseolus vulgaris L. J. Amer. Soc. Hort. Sci. 111:807-810. Weaver, M.L. and H. Timm. 1988. Influence of temperature and plant water status on pollen viability in beans. J. Amer. Soc. Hort. Sci. 113231—35. SUMMARY AND CONCLUSIONS The objectives of this study were to identify genotypic om_ tmnEsoao to 966883 macaroxm mom E mmmcoco 6:55 .N .mE TIV xoo to mEfi a: m— \L m: mm A; m— N__ Z 0— m w r — n F bl Li n FL [r b h F L F - — n O ) l I S / x x m m I I. w ( e I I m.v S c 1 I J S n O l. O r 1 w w LL I: C .OIIDI u I 1w SD... 10 /O n x 1 pU m w - - Zm .l _ 1. m I , we 0 U\ m 1 1N— 0 It I T S r. I co:o__E_mm< I u x mm I Ummmobm QTAO x @— APPENDIX B Effect of leaf age on photosynthetic rate in cucumber leaves 110 Effect of leaf age on photosynthetic rate in cucumber leaves Table 1. Net photosynthetic rates for leaves at different node pOSitions on pickling cucumber plants. 2 C02 assimilati n rate Leaf position (umol C02/dm -hr) 3 17.2 6 19.1 9 20.9 12 16.5 F-significance NS 2. Measurements were made on the 3rd, 6th, 9th and 12th leaf from the shoot apex of well watered cucumber plants in the greenhouse under light saturating conditions. NS.Not significant at the 5% probability level. 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